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.

.github/scripts/install_deps.sh

@@ -30,12 +30,12 @@ if [[ $os == Linux ]]; then
   fi
 
   if [[ -n $TEST ]]; then
-    sudo apt-get install -y locales-all cpanminus attr libattr1-dev gdb
+    sudo apt-get install -y locales-all cpanminus attr libattr1-dev gdb fswatch
   fi
 elif [[ $os == Darwin ]]; then
   brew update --quiet
   brew install ninja
   if [[ -n $TEST ]]; then
-    brew install cpanminus
+    brew install cpanminus fswatch
   fi
 fi