feat(lsp): add fswatch watchfunc backend

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.

test/functional/lua/watch_spec.lua

@@ -21,6 +21,15 @@ describe('vim._watch', function()
 
   local function run(watchfunc)
     it('detects file changes (watchfunc=' .. watchfunc .. '())', function()
+      if watchfunc == 'fswatch' then
+        skip(is_os('mac'), 'flaky test on mac')
+        skip(
+          not is_ci() and helpers.fn.executable('fswatch') == 0,
+          'fswatch not installed and not on CI'
+        )
+        skip(is_os('win'), 'not supported on windows')
+      end
+
       if watchfunc == 'watch' then
         skip(is_os('bsd'), 'Stopped working on bsd after 3ca967387c49c754561c3b11a574797504d40f38')
       else
@@ -95,6 +104,7 @@ describe('vim._watch', function()
 
       vim.uv.sleep(100)
       touch(watched_path)
+      vim.uv.sleep(100)
       os.remove(watched_path)
       vim.uv.sleep(100)
 
@@ -113,5 +123,5 @@ describe('vim._watch', function()
 
   run('watch')
   run('watchdirs')
+  run('fswatch')
 end)
-