Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/yaml/language-configuration.json

@@ -0,0 +1,35 @@
+{
+	"comments": {
+		"lineComment": "#"
+	},
+	"brackets": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"]
+	],
+	"autoClosingPairs": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"],
+		["\"", "\""],
+		["'", "'"]
+	],
+	"surroundingPairs": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"],
+		["\"", "\""],
+		["'", "'"]
+	],
+	"folding": {
+		"offSide": true,
+		"markers": {
+			"start": "^\\s*#\\s*region\\b",
+			"end": "^\\s*#\\s*endregion\\b"
+		}
+	},
+	"indentationRules": {
+		"increaseIndentPattern": "^\\s*.*(:|-) ?(&\\w+)?(\\{[^}\"']*|\\([^)\"']*)?$",
+		"decreaseIndentPattern": "^\\s+\\}$"
+	}
+}