Remove readonly from Value properties.

source/value.ts

@@ -2,7 +2,7 @@ import browser from "webextension-polyfill";
 
 export type StorageArea = browser.Storage.StorageArea;
 
-type ValueOptions<T> = {
+export type ValueOptions<T> = {
   /** A function to convert a string to the type `T`. */
   deserialize: (input: string) => T;
   /** The key to use for storage. */
@@ -35,10 +35,10 @@ export async function createValue<T>(
 type Props<T> = Required<ValueOptions<T>>;
 
 export class Value<T> implements Props<T> {
-  public readonly deserialize: Props<T>["deserialize"];
-  public readonly key: Props<T>["key"];
-  public readonly serialize: Props<T>["serialize"];
-  public readonly storage: Props<T>["storage"];
+  public deserialize: Props<T>["deserialize"];
+  public key: Props<T>["key"];
+  public serialize: Props<T>["serialize"];
+  public storage: Props<T>["storage"];
 
   private inner: Props<T>["value"];