Support for embedded Markdown comments

Summary:
Erlang/OTP 27 introduces Markdown comments. While this makes it in theory simpler to author in-code documentation, the VS Code extension does not yet recognize comments as Markdown.

This change makes VS Code recognize doc blocks as Markdown, using the concept of an [embedded language](https://code.visualstudio.com/api/language-extensions/syntax-highlight-guide#embedded-languages).

This provides features such as:

* Syntax highlighting for italic, bold, code blocks, etc
* Matching parentheses
* Ability to embed Erlang code blocks with syntax highlighting
* Etc

While this simply works(TM), it makes the doc block too evident and distracting:

{F1978476087}

To preserve the look-and-feel of the old edoc-style comment, we override the default colour for the embedded markdown block. This seems to work fine in both light and dark mode and can always be overridden by a user using the `settings.json` file:

```
"editor.tokenColorCustomizations": {
        "textMateRules": [
            {
                "scope": "meta.embedded.block.markdown",
                "settings": {
                    "foreground": "MY_FAVOURITE_COLOUR"
                }
            }
        ],
    }
```

The change is being upstreamed [here](erlang-ls/grammar#18).

Reviewed By: alanz

Differential Revision: D75152988

fbshipit-source-id: 3922007c13678be99fb27c99e53f8597c7dcba8f

Author: robertoaloi

editors/code/package.json

@@ -330,6 +330,18 @@
         }
       }
     },
+    "configurationDefaults": {
+      "editor.tokenColorCustomizations": {
+        "textMateRules": [
+          {
+            "scope": "meta.embedded.block.markdown",
+            "settings": {
+                "foreground": "#6A9955"
+            }
+        }
+        ]
+      }
+    },
     "semanticTokenTypes": [],
     "semanticTokenModifiers": [
       {

editors/code/third-party/grammar/Erlang.plist

@@ -604,15 +604,13 @@
 			</dict>
 			<key>name</key>
 			<string>meta.directive.doc.erlang</string>
+			<key>contentName</key>
+			<string>meta.embedded.block.markdown</string>
 			<key>patterns</key>
 			<array>
 				<dict>
-					<key>begin</key>
-					<string>\G</string>
-					<key>contentName</key>
-					<string>comment.block.documentation.erlang</string>
-					<key>while</key>
-					<string>(^)(?!\s*(\5))</string>
+					<key>include</key>
+					<string>text.html.markdown</string>
 				</dict>
 			</array>
 		</dict>