feat: return empty response to textDocument/formatting if no changes

Do not send "edits" if the content remains unchanged after LSP
formatting. Otherwise, Neovim reports an unsaved buffer even when no
actual changes were made

Author: danielleontiev

script/core/formatting.lua

@@ -21,6 +21,10 @@ return function(uri, options)
         return
     end
 
+    if text == formattedText then
+        return
+    end
+
     return {
         {
             start = state.ast.start,