Implement invocation strategy config
Fixes https://github.com/rust-lang/rust-analyzer/issues/10793
This allows to change how we run build scripts (and `checkOnSave`), exposing two configs:
- `once`: run the specified command once in the project root (the working dir of the server)
- `per_workspace`: run the specified command per workspace in the corresponding workspace
This also applies to `checkOnSave` likewise, though `once_in_root` is useless there currently, due to https://github.com/rust-lang/cargo/issues/11007
a77ac93b2a