Add the build and web-ext files.

source/build.ts

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+// Import native Node libraries.
+import path from "node:path";
+import process from "node:process";
+import fsp from "node:fs/promises";
+
+// Import Esbuild and associated plugins.
+import esbuild from "esbuild";
+import copyPlugin from "esbuild-copy-static-files";
+import {sassPlugin} from "esbuild-sass-plugin";
+
+// Import PostCSS and associated plugins.
+import cssnano from "cssnano";
+import postcss from "postcss";
+
+// Import local functions.
+import {createManifest} from "./manifest.js";
+import {createWebExtConfig} from "./web-ext.js";
+
+/**
+ * Create an absolute path from a given relative one, using the directory
+ * this file is located in as the base.
+ *
+ * @param relative The relative path to make absolute.
+ * @returns The absolute path.
+ */
+function toAbsolutePath(relative: string): string {
+  return new URL(relative, import.meta.url).pathname;
+}
+
+// Create variables based on the environment.
+const browser = process.env.BROWSER ?? "firefox";
+const dev = process.env.NODE_ENV === "development";
+const test = process.env.TEST === "true";
+const watch = process.env.WATCH === "true";
+
+// Create absolute paths to various directories.
+const buildDir = toAbsolutePath("build");
+const outDir = path.join(buildDir, browser);
+const sourceDir = toAbsolutePath("source");
+
+// Ensure that the output directory exists.
+await fsp.mkdir(outDir, {recursive: true});
+
+// Write the WebExtension manifest file.
+await fsp.writeFile(
+  path.join(outDir, "manifest.json"),
+  JSON.stringify(createManifest(browser)),
+);
+
+// Write the web-ext configuration file.
+await fsp.writeFile(
+  path.join(buildDir, `web-ext-${browser}.json`),
+  JSON.stringify(createWebExtConfig(browser, buildDir, dev, outDir)),
+);
+
+const cssProcessor = postcss([cssnano()]);
+
+const options: esbuild.BuildOptions = {
+  bundle: true,
+  // Define variables to be replaced in the code. Note that these are replaced
+  // "as is" and so we have to stringify them as JSON, otherwise a string won't
+  // have its quotes for example.
+  define: {
+    $browser: JSON.stringify(browser),
+    $dev: JSON.stringify(dev),
+    $test: JSON.stringify(test),
+  },
+  entryPoints: [
+    path.join(sourceDir, "background/setup.ts"),
+    path.join(sourceDir, "options/setup.tsx"),
+  ],
+  format: "esm",
+  logLevel: "info",
+  minify: !dev,
+  outdir: outDir,
+  plugins: [
+    // Copy all files from `source/assets/` to the output directory.
+    copyPlugin({src: path.join(sourceDir, "assets/"), dest: outDir}),
+
+    // Compile SCSS to CSS.
+    sassPlugin({
+      type: "style",
+      async transform(source) {
+        // In development, don't do any extra processing.
+        if (dev) {
+          return source;
+        }
+
+        // But in production, run the CSS through PostCSS.
+        const {css} = await cssProcessor.process(source, {from: undefined});
+        return css;
+      },
+    }),
+  ],
+  // Link sourcemaps in development but omit them in production.
+  sourcemap: dev ? "linked" : false,
+  // Currently code splitting can't be used because we use ES modules and
+  // Firefox doesn't run the background script with `type="module"`.
+  // Once Firefox properly supports Manifest V3 this should be possible though.
+  splitting: false,
+  // Target ES2022, and the first Chromium and Firefox releases from 2022.
+  target: ["es2022", "chrome97", "firefox102"],
+  treeShaking: true,
+};
+
+if (watch) {
+  const context = await esbuild.context(options);
+  await context.watch();
+} else {
+  await esbuild.build(options);
+}

source/web-ext.ts

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+import path from "node:path";
+
+/**
+ * Barebones type definition for web-ext configuration.
+ *
+ * Since web-ext doesn't export any types this is done by ourselves. The keys
+ * mostly follow a camelCased version of the CLI options
+ * (ie. --start-url becomes startUrl).
+ */
+type WebExtConfig = {
+  artifactsDir: string;
+  sourceDir: string;
+  verbose?: boolean;
+
+  build: {
+    filename: string;
+    overwriteDest: boolean;
+  };
+
+  run: {
+    browserConsole: boolean;
+    firefoxProfile: string;
+    keepProfileChanges: boolean;
+    profileCreateIfMissing: boolean;
+    startUrl: string[];
+    target: string[];
+  };
+};
+
+/**
+ * Create the web-ext configuration.
+ *
+ * @param browser The browser target ("firefox" or "chromium").
+ * @param buildDir The path to the build directory.
+ * @param dev Is this for development or production.
+ * @param outDir The path to the output directory.
+ * @returns The configuration for web-ext.
+ */
+export function createWebExtConfig(
+  browser: string,
+  buildDir: string,
+  dev: boolean,
+  outDir: string,
+): WebExtConfig {
+  const config: WebExtConfig = {
+    artifactsDir: path.join(buildDir, "artifacts"),
+    sourceDir: outDir,
+
+    build: {
+      filename: `{name}-{version}-${browser}.zip`,
+      overwriteDest: true,
+    },
+
+    run: {
+      browserConsole: dev,
+      firefoxProfile: path.join(buildDir, "firefox-profile/"),
+      keepProfileChanges: true,
+      profileCreateIfMissing: true,
+      startUrl: [],
+      target: [],
+    },
+  };
+
+  if (browser === "firefox") {
+    config.run.startUrl.push("about:debugging#/runtime/this-firefox");
+    config.run.target.push("firefox-desktop");
+  } else if (browser === "chromium") {
+    config.run.startUrl.push("chrome://extensions/");
+    config.run.target.push("chromium");
+  } else {
+    throw new Error(`Unknown target browser: ${browser}`);
+  }
+
+  return config;
+}