Add a HideTopicMatcher type guard.

source/storage/hide-topics.ts

@@ -12,6 +12,14 @@ export enum HideTopicMatcher {
   UserLabelEquals = "user-label-equals",
 }
 
+/**
+ * Type guard check to see if a string is a valid {@link HideTopicMatcher}.
+ * @param input The string to check.
+ */
+export function isHideTopicMatcher(input: string): input is HideTopicMatcher {
+  return Object.values(HideTopicMatcher).includes(input as HideTopicMatcher);
+}
+
 /**
  * The predicate for whether a topic should be hidden or not.
  */