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.

runtime/lua/vim/_watch.lua

@@ -222,5 +222,81 @@ function M.watchdirs(path, opts, callback)
   return cancel
 end
 
-return M
+--- @param data string
+--- @param opts vim._watch.Opts?
+--- @param callback vim._watch.Callback
+local function fswatch_output_handler(data, opts, callback)
+  local d = vim.split(data, '%s+')
 
+  -- only consider the last reported event
+  local fullpath, event = d[1], d[#d]
+
+  if skip(fullpath, opts) then
+    return
+  end
+
+  --- @type integer
+  local change_type
+
+  if event == 'Created' then
+    change_type = M.FileChangeType.Created
+  elseif event == 'Removed' then
+    change_type = M.FileChangeType.Deleted
+  elseif event == 'Updated' then
+    change_type = M.FileChangeType.Changed
+  elseif event == 'Renamed' then
+    local _, staterr, staterrname = uv.fs_stat(fullpath)
+    if staterrname == 'ENOENT' then
+      change_type = M.FileChangeType.Deleted
+    else
+      assert(not staterr, staterr)
+      change_type = M.FileChangeType.Created
+    end
+  end
+
+  if change_type then
+    callback(fullpath, change_type)
+  end
+end
+
+--- @param path string The path to watch. Must refer to a directory.
+--- @param opts vim._watch.Opts?
+--- @param callback vim._watch.Callback Callback for new events
+--- @return fun() cancel Stops the watcher
+function M.fswatch(path, opts, callback)
+  -- debounce isn't the same as latency but close enough
+  local latency = 0.5 -- seconds
+  if opts and opts.debounce then
+    latency = opts.debounce / 1000
+  end
+
+  local obj = vim.system({
+    'fswatch',
+    '--event=Created',
+    '--event=Removed',
+    '--event=Updated',
+    '--event=Renamed',
+    '--event-flags',
+    '--recursive',
+    '--latency=' .. tostring(latency),
+    '--exclude',
+    '/.git/',
+    path,
+  }, {
+    stdout = function(err, data)
+      if err then
+        error(err)
+      end
+
+      for line in vim.gsplit(data or '', '\n', { plain = true, trimempty = true }) do
+        fswatch_output_handler(line, opts, callback)
+      end
+    end,
+  })
+
+  return function()
+    obj:kill(2)
+  end
+end
+
+return M