docs: Validate JSON snippets (settings, keymap, tasks, etc) (#40043)

Closes #ISSUE

Release Notes:

- N/A *or* Added/Fixed/Improved ...

docs/src/languages/dart.md

@@ -22,7 +22,7 @@ dart --version
 
 If you would like to use a specific dart binary or use dart via FVM you can specify the `dart` binary in your Zed settings.jsons file:
 
-```json
+```json [settings]
 {
   "lsp": {
     "dart": {
@@ -39,7 +39,7 @@ If you would like to use a specific dart binary or use dart via FVM you can spec
 
 Dart by-default uses a very conservative maximum line length (80). If you would like the dart LSP to permit a longer line length when auto-formatting, add the following to your Zed settings.json:
 
-```json
+```json [settings]
 {
   "lsp": {
     "dart": {