Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/csharp/language-configuration.json

@@ -0,0 +1,32 @@
+{
+	"comments": {
+		"lineComment": "//",
+		"blockComment": ["/*", "*/"]
+	},
+	"brackets": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"]
+	],
+	"autoClosingPairs": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"],
+		{ "open": "'", "close": "'", "notIn": ["string", "comment"] },
+		{ "open": "\"", "close": "\"", "notIn": ["string", "comment"] }
+	],
+	"surroundingPairs": [
+		["{", "}"],
+		["[", "]"],
+		["(", ")"],
+		["<", ">"],
+		["'", "'"],
+		["\"", "\""]
+	],
+	"folding": {
+		"markers": {
+			"start": "^\\s*#region\\b",
+			"end": "^\\s*#endregion\\b"
+		}
+	}
+}