Add support for clangd's switchSourceHeader command

[bciobanu]

clangd supports several extensions outside the Language Server Protocol specification. One of them is the functionality to determine the corresponding source/header file from another file: https://clangd.llvm.org/extensions#switch-between-sourceheader.

In this PR I add support for that request. Tested:

• [x] cargo test
• [x] Integration with helix-editor/helix