Enable Typescript strictFunctionTypes (#32911)

1. Enable
[strictFunctionTypes](https://www.typescriptlang.org/tsconfig/#strictFunctionTypes)
2. Introduce `DOMEvent` helper type which sets `e.target`. Surely not
totally correct with that `Partial` but seems to work.
3. Various type-related refactors, change objects in
`eventsource.sharedworker.ts` to `Map`.

web_src/js/features/repo-wiki.ts

@@ -13,7 +13,7 @@ async function initRepoWikiFormEditor() {
   let editor: ComboMarkdownEditor;
 
   let renderRequesting = false;
-  let lastContent;
+  let lastContent: string = '';
   const renderEasyMDEPreview = async function () {
     if (renderRequesting) return;