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`.
Fixes 79 typescript errors. Discovered at least two bugs in
`notifications.ts`, and I'm pretty sure this feature was at least
partially broken and may still be, I don't really know how to test it.
After this, only like ~10 typescript errors remain in the codebase but
those are harder to solve.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
