Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/html/language-configuration.json

@@ -0,0 +1,33 @@
+{
+	"comments": {
+		"blockComment": [ "<!--", "-->" ]
+	},
+	"brackets": [
+		["<!--", "-->"],
+		["<", ">"],
+		["{", "}"],
+		["(", ")"]
+	],
+	"autoClosingPairs": [
+		{ "open": "{", "close": "}"},
+		{ "open": "[", "close": "]"},
+		{ "open": "(", "close": ")" },
+		{ "open": "'", "close": "'" },
+		{ "open": "\"", "close": "\"" },
+		{ "open": "<!--", "close": "-->", "notIn": [ "comment", "string" ]}
+	],
+	"surroundingPairs": [
+		{ "open": "'", "close": "'" },
+		{ "open": "\"", "close": "\"" },
+		{ "open": "{", "close": "}"},
+		{ "open": "[", "close": "]"},
+		{ "open": "(", "close": ")" },
+		{ "open": "<", "close": ">" }
+	],
+	"folding": {
+		"markers": {
+			"start": "^\\s*<!--\\s*#region\\b.*-->",
+			"end": "^\\s*<!--\\s*#endregion\\b.*-->"
+		}
+	}
+}