Merge #7657

7657: utf8 r=matklad a=matklad

- Prepare for utf-8 offsets
- reduce code duplication in tests
- Make utf8 default, implement utf16 in terms of it
- Make it easy to add additional context for offset conversion
- Implement utf8 offsets

closes #7453

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

diff --cc crates/rust-analyzer/src/config.rs
Simple merge

diff --cc crates/rust-analyzer/src/to_proto.rs
Simple merge