Release wasm binaries

For adding RA to the playground, it would be nice to have wasm binaries in releases in addition to linux, windows, vsix and ... so the building logic can be separated from playground and we can select between RA versions in playground, which enables using it to share code in issues and see behavior of latest RA instantly (like how playground is used in rustc and clippy).

I see three options:

1. Do nothing. Playground can use a specific RA version, and bump it manually, occasionally (for example once a year).
2. Declare interface in rust-analyzer-wasm repo as stable (after some changes) and build wasm binary for each release with that interface.
3. Compile RA (after stripping unnecessary parts) as is to WASI, so LSP will become the interface. It would make the binary bigger probably, but it will enable to use it in web extension easily. Web extension should be possible with 2, but it needs a completely different code for the extension. It can also be used for targets that we don't have build officially, but they will probably prefer building from source to using 10x slower wasm.

@shepmaster is it ok to import RA wasm from third party in playground? If it isn't and we need to self host it, we can go with 1 and defer releasing wasm binaries to when another usecase appears for them.

No file or test is named. Start by reviewing the release/build logic and the rust-analyzer-wasm interface discussed here, then resolve which proposed approach the project should pursue; done means the chosen WASM release approach supports the intended playground or web-extension use case.