lsp: Add support for clangd's `inactiveRegions` extension (#26146)

Closes #13089 

Here we use `experimental` to advertise our support for
`inactiveRegions`. Note that clangd does not currently have a stable
release that reads the `experimental` object (PR
https://github.com/llvm/llvm-project/pull/116531), this can be tested
with one of clangd's recent "unstable snapshots" in their
[releases](https://github.com/clangd/clangd/releases).

Release Notes:

- Added support for clangd's `inactiveRegions` extension.

![Screen Recording 2025-03-05 at 22 39
58](https://github.com/user-attachments/assets/ceade8bd-4d8e-43c3-9520-ad44efa50d2f)

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Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>