Merge #6761

Merge #6761

6761: Make config.rs a single source of truth for configuration. r=matklad a=matklad

Configuration is editor-independent. For this reason, we pick
JSON-schema as the repr of the source of truth. We do specify it using
rust-macros and some quick&dirty hackery though.

The idea for syncing truth with package.json is to just do that
manually, but there's a test to check that they are actually synced.

I'll add something like `rust-analyzer --config-schema` in a follow-up
commit.

Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>