feat: typed context draft

[olehmisar]

Svelte lacks strong types in it's context functions. This PR is a draft of what this could look like. More details in this issue: #8941.

1. I tried to preserve backwards compatibility by replacing key: any with key: {}. It should be the same as before except undefined and null. But I don't think someone used undefined or null as keys anyway.

2. There are 2 commented out tests because I didn't want to think how to fix those edge cases because IDK if this PR will even be merged.

3. There is a nasty edge case with getContext(Symbol()), which returns T | undefined instead of T. But I believe if # 2 is resolved, this will be resolved too.

4. Also, I moved getContext types to public.d.ts because JSDoc doesn't support function overloads(as far as I am aware).

Is it possible for this to be merged in Svelte 5?

Svelte 5 rewrite

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[Rich-Harris]

Thank you for this — per #11042 (comment), we're going to revisit this post-5.0, so for the sake of clearing out the PR queue I'm going to close this for now