Allow formatting of unsaved buffers with prettier (#12095)

This fixes #4529 by allowing unsaved buffers to be formatted with
prettier.

Steps to do that:

1. Create a new buffer
2. Set language for the buffer (e.g.: `language selector: toggle` and
JSON)
3. In settings, set prettier parser for language (can't be inferred,
since we don't have filename) and allow formatting with prettier:

   ```json
   {
     "languages": {
       "JSON": {
         "prettier": {
           "allowed": true,
           "parser": "json"
         }
       }
     }
   }
   ```

4. Use `editor: format`

Release Notes:

- Added ability to format unsaved buffers with Prettier. Requirement is
to set a Prettier parser in the user settings. Example for JSON: `{
"languages": { "JSON": { "prettier": { "allowed": true, "parser": "json"
} } } }` ([#4529](https://github.com/zed-industries/zed/issues/4529)).

Demo:


https://github.com/zed-industries/zed/assets/1185253/d24e490b-2e2c-4a5d-95a8-fc8675523780

crates/prettier/src/prettier_server.js

@@ -186,11 +186,17 @@ async function handleMessage(message, prettier) {
     }
 
     let resolvedConfig = {};
-    if (params.options.filepath !== undefined) {
+    if (params.options.filepath) {
       resolvedConfig =
         (await prettier.prettier.resolveConfig(params.options.filepath)) || {};
     }
 
+    // Marking the params.options.filepath as undefined makes
+    // prettier.format() work even if no filepath is set.
+    if (params.options.filepath === null) {
+      params.options.filepath = undefined;
+    }
+
     const plugins =
       Array.isArray(resolvedConfig?.plugins) &&
       resolvedConfig.plugins.length > 0