feat: vscode: Support opening local documentation if available
This PR implements the VS code support for opening local documentation (server side support was already implemented in #14662).
[local_docs.webm](https://github.com/rust-lang/rust-analyzer/assets/9659253/715b84dd-4f14-4ba0-a904-749b847eb3d5)
Displaying local instead of web docs can have many benefits:
- the web version may have different features enabled than locally selected
- the standard library may be a different version than is available online
- the user may not be online and therefore cannot access the web documentation
- the documentation may not be available online at all, for example because it is for a new feature in a library the user is currently developing
If the documentation is not available locally, the extension still falls back to the web version.
Closes#12867.
-----
If my implementation isn't really idiomatic TypeScript: Sorry, I'm not much of a TypeScript developer. I am open to feedback, however.
8a2331450a