[clangd] Support for standard inlayHint protocol

- Make clangd's internal representation more aligned with the standard.
  We keep range and extra inlayhint kinds around, but don't serialize
  them on standard version.
- Have custom serialization for extension (ugly, but going to go away).
- Support both versions until clangd-17.
- Don't advertise extension if client has support for standard
  implementation.
- Log a warning at startup about extension being deprecated, if client
  doesn't have support.

Differential Revision: https://reviews.llvm.org/D125228