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-search.ts

@@ -1,8 +1,10 @@
+import type {DOMEvent} from '../utils/dom.ts';
+
 export function initRepositorySearch() {
   const repositorySearchForm = document.querySelector<HTMLFormElement>('#repo-search-form');
   if (!repositorySearchForm) return;
 
-  repositorySearchForm.addEventListener('change', (e: Event & {target: HTMLFormElement}) => {
+  repositorySearchForm.addEventListener('change', (e: DOMEvent<Event, HTMLInputElement>) => {
     e.preventDefault();
 
     const params = new URLSearchParams();