Add basic mermaid rendering support in core

For #257761

Ports over extension sample + a few improvements to core

extensions/mermaid-chat-features/chat-webview-src/tsconfig.json

@@ -0,0 +1,12 @@
+{
+	"extends": "../../tsconfig.base.json",
+	"compilerOptions": {
+		"outDir": "./dist/",
+		"jsx": "react",
+		"lib": [
+			"ES2024",
+			"DOM",
+			"DOM.Iterable"
+		]
+	}
+}