Add basic mermaid rendering support in core

For #257761

Ports over extension sample + a few improvements to core

extensions/mermaid-chat-features/tsconfig.json

@@ -0,0 +1,15 @@
+{
+	"extends": "../tsconfig.base.json",
+	"compilerOptions": {
+		"outDir": "./out",
+		"types": []
+	},
+	"include": [
+		"src/**/*",
+		"../../src/vscode-dts/vscode.d.ts",
+		"../../src/vscode-dts/vscode.proposed.chatOutputRenderer.d.ts",
+		"../../src/vscode-dts/vscode.proposed.chatParticipantAdditions.d.ts",
+		"../../src/vscode-dts/vscode.proposed.languageModelThinkingPart.d.ts",
+		"../../src/vscode-dts/vscode.proposed.chatParticipantPrivate.d.ts"
+	]
+}