Add "proposal" topic to WebAssembly proposal repos

[binji]

See the comment here: #950 (comment)

It would be useful to be able to track all WebAssembly proposals with one search query, it seems like we could add a proposal topic to the repo, then use org:WebAssembly topic:proposal to find these.

[jfbastien]

Good idea!
Done: https://github.com/search?q=topic%3Aproposal+org%3AWebAssembly&type=Repositories

[trusktr]

Looks like this is not in sync with the proposals repo.

[dtig]

This should work better now, we may be missing some proposals that are already merged and marked as read-only.