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`.

tsconfig.json

@@ -22,6 +22,7 @@
     "verbatimModuleSyntax": true,
     "stripInternal": true,
     "strict": false,
+    "strictFunctionTypes": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "noPropertyAccessFromIndexSignature": false,