Merge #3247

3247: Improve RA version display r=matklad a=edwin0cheng

There are 2 problems of current implementation for displaying current version of RA binary:

1. If that binary is coming from built by source, the `REV` may not be updated somehow. (See discussion in [Zuilp](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/vscode.20version.20in.20console))

2. We must go through the VSCode debugger console to see the output of `console.log`.

This PR implemented a new VSCode command "Show RA Version" to display the version, which  fixed the first problem.  And added some `rerun-if` flags in `build.rs` to fix the second one.

Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>