Problem:
  vim._watch.watchdirs has terrible performance.

Solution:
  - On linux use fswatch as a watcher backend if available.

  - Add File watcher section to health:vim.lsp. Warn if watchfunc is
    libuv-poll.

- Rename watch.poll to watch.watchdirs
- Unify how include and exclude is applied
- Improve type hints

Problem: Some LSP servers return `textDocument/documentLink` responses
         containing file URIs with line/column numbers in the fragment.
         `vim.uri_to_fname` returns invalid file names for these URIs.

Solution: Remove the URI fragment from file URIs.

Currently, highlight.on_yank() does buffer-local highlighting, this PR
makes it window scoped.

Also fix the problem that when yanking in a buffer, moving to another
buffer, and yanking before the original buffer highlight disappears, the
original buffer highlight won't disappear on timeout.

Problem:
`vim.ui.open` uses `wslview`, which is slow and require a package from external PPA:
https://wslutiliti.es/wslu/install.html#ubuntu

Solution:
Use `explorer.exe` instead. WSL supports it by default:
https://learn.microsoft.com/en-us/windows/wsl/filesystems#view-your-current-directory-in-windows-file-explorer

Problem:  Error when parsing v:lua in a ternary expression.
Solution: Set rettv->v_type for v:lua even if not evaluating.

and for return value of nlua_exec/nlua_call_ref, as this uses
the same family of functions.

NB: the handling of luaref:s is a bit of a mess.
add api_luarefs_free_XX functions as a stop-gap as refactoring
luarefs is a can of worms for another PR:s.

as a minor feature/bug-fix, nvim_buf_call and nvim_win_call now preserves
arbitrary return values.

Problem: Loading `vim.fs` via the `vim.loader` Lua package loader will
result in a stack overflow due to a cyclic dependency. This may happen
when the `vim.fs` module isn't byte-compiled, i.e. when `--luamod-dev`
is used (#27413).

Solution: `vim.loader` depends on `vim.fs`. Therefore `vim.fs` should
be loaded in advance.