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

Closes #ISSUE

Release Notes:

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

docs/src/languages/elixir.md

@@ -21,7 +21,7 @@ The Elixir extension offers language server support for `expert`, `elixir-ls`, `
 
 To switch to `expert`, add the following to your `settings.json`:
 
-```json
+```json [settings]
   "languages": {
     "Elixir": {
       "language_servers": ["expert", "!elixir-ls", "!next-ls", "!lexical", "..."]
@@ -36,7 +36,7 @@ To switch to `expert`, add the following to your `settings.json`:
 
 To switch to `next-ls`, add the following to your `settings.json`:
 
-```json
+```json [settings]
   "languages": {
     "Elixir": {
       "language_servers": ["next-ls", "!expert", "!elixir-ls", "!lexical", "..."]
@@ -51,7 +51,7 @@ To switch to `next-ls`, add the following to your `settings.json`:
 
 To switch to `lexical`, add the following to your `settings.json`:
 
-```json
+```json [settings]
   "languages": {
     "Elixir": {
       "language_servers": ["lexical", "!expert", "!elixir-ls", "!next-ls", "..."]
@@ -84,11 +84,12 @@ brew install elixir-ls
 
 If you prefer to format your code with [Mix](https://hexdocs.pm/mix/Mix.html), use the following snippet in your `settings.json` file to configure it as an external formatter. Formatting will occur on file save.
 
-```json
+```json [settings]
 {
   "languages": {
     "Elixir": {
-      "format_on_save": {
+      "format_on_save": "on",
+      "formatter": {
         "external": {
           "command": "mix",
           "arguments": ["format", "--stdin-filename", "{buffer_path}", "-"]
@@ -105,7 +106,7 @@ You can pass additional elixir-ls workspace configuration options via lsp settin
 
 The following example disables dialyzer:
 
-```json
+```json [settings]
   "lsp": {
     "elixir-ls": {
       "settings": {