Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/markdown-basics/language-configuration.json

@@ -0,0 +1,50 @@
+{
+	"comments": {
+		// symbols used for start and end a block comment. Remove this entry if your language does not support block comments
+		"blockComment": [
+			"<!--",
+			"-->"
+		]
+	},
+	// symbols used as brackets
+	"brackets": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"]
+	],
+	"autoClosingPairs": [
+		{
+			"open": "{",
+			"close": "}"
+		},
+		{
+			"open": "[",
+			"close": "]"
+		},
+		{
+			"open": "(",
+			"close": ")"
+		},
+		{
+			"open": "<",
+			"close": ">",
+			"notIn": [
+				"string"
+			]
+		}
+	],
+	"surroundingPairs": [
+		["(", ")"],
+		["[", "]"],
+		["`", "`"],
+		["_", "_"],
+		["*", "*"]
+	],
+	"folding": {
+		"offSide": true,
+		"markers": {
+			"start": "^\\s*<!--\\s*#?region\\b.*-->",
+			"end": "^\\s*<!--\\s*#?endregion\\b.*-->"
+		}
+	}
+}