feat(autoread): use filewatchers for OS-driven change detection #37971

Problem:
The 'autoread' option only checks for file changes reactively — on
FocusGained, :checktime, CmdlineEnter, etc. — by polling timestamps.
External changes are not detected until the user interacts with Neovim.

Solution:
Add a core module (runtime/lua/nvim/autoread.lua) enabled from
runtime/plugin/autoread.lua that watches each buffer's file using
vim._watch.watch() (libuv fs_event). On change detection it calls
:checktime, which invokes the existing buf_check_timestamp() logic
for reload/prompt handling. Watchers are managed via autocmds tied
to buffer lifecycle events and respect the 'autoread' option (global
and buffer-local).

runtime/lua/vim/_meta/options.gen.lua

@@ -172,7 +172,8 @@ vim.bo.ai = vim.bo.autoindent
 --- it has not been changed inside of Vim, automatically read it again.
 --- When the file has been deleted this is not done, so you have the text
 --- from before it was deleted.  When it appears again then it is read.
---- `timestamp`
+--- Nvim uses file system watchers to detect changes in real-time for all
+--- loaded buffers; see `timestamp` for details.
 --- If this option has a local value, use this command to switch back to
 --- using the global value:
 ---