feat: add option to disable ftplugin (#417)

Author: obemu

ftplugin/dart/init.lua

@@ -1,3 +1,9 @@
+-- See section 'DISABLING' in :h ftplugin
+if vim.b.flutter_tools_did_ftplugin then
+  return
+end
+vim.b.flutter_tools_did_ftplugin = 1
+
 require("flutter-tools.lsp").attach()
 
 vim.opt_local.comments = [[sO:*\ -,mO:*\ \ ,exO:*/,s1:/*,mb:*,ex:*/,:///,://]]