Add basic mermaid rendering support in core

For #257761

Ports over extension sample + a few improvements to core

extensions/mermaid-chat-features/package.json

@@ -0,0 +1,75 @@
+{
+  "name": "marmaid-chat-features",
+  "displayName": "%displayName%",
+  "description": "%description%",
+  "version": "1.0.0",
+  "publisher": "vscode",
+  "license": "MIT",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/microsoft/vscode.git"
+  },
+  "aiKey": "0c6ae279ed8443289764825290e4f9e2-1a736e7c-1324-4338-be46-fc2a58ae4d14-7255",
+  "engines": {
+    "vscode": "^1.104.0"
+  },
+  "enabledApiProposals": [
+    "chatOutputRenderer"
+  ],
+  "capabilities": {
+    "virtualWorkspaces": true,
+    "untrustedWorkspaces": {
+      "supported": true
+    }
+  },
+  "main": "./out/extension",
+  "browser": "./dist/browser/extension",
+  "activationEvents": [],
+  "contributes": {
+    "chatOutputRenderers": [
+      {
+        "viewType": "vscode.chatMermaidDiagram",
+        "mimeTypes": [
+          "text/vnd.mermaid"
+        ]
+      }
+    ],
+    "languageModelTools": [
+      {
+        "name": "renderMermaidDiagram",
+        "displayName": "Mermaid Renderer",
+        "toolReferenceName": "renderMermaidDiagram",
+        "canBeReferencedInPrompt": true,
+        "modelDescription": "Renders a Mermaid diagram from Mermaid.js markup.",
+        "userDescription": "Render a Mermaid.js diagrams from markup.",
+        "inputSchema": {
+          "type": "object",
+          "properties": {
+            "markup": {
+              "type": "string",
+              "description": "The mermaid diagram markup to render as a Mermaid diagram. This should only be the markup of the diagram. Do not include a wrapping code block."
+            }
+          }
+        }
+      }
+    ]
+  },
+  "scripts": {
+    "compile": "gulp compile-extension:mermaid-chat-features && npm run build-chat-webview",
+    "watch": "npm run build-chat-webview && gulp watch-extension:mermaid-chat-features",
+    "vscode:prepublish": "npm run build-ext && npm run build-chat-webview",
+    "build-ext": "node ../../node_modules/gulp/bin/gulp.js --gulpfile ../../build/gulpfile.extensions.js compile-extension:mermaid-chat-features ./tsconfig.json",
+    "build-chat-webview": "node ./esbuild-chat-webview.mjs",
+    "compile-web": "npx webpack-cli --config extension-browser.webpack.config --mode none",
+    "watch-web": "npx webpack-cli --config extension-browser.webpack.config --mode none --watch --info-verbosity verbose"
+  },
+  "devDependencies": {
+    "@types/jsdom": "^21.1.7",
+    "@types/node": "^22"
+  },
+  "dependencies": {
+    "dompurify": "^3.2.6",
+    "jsdom": "^26.1.0",
+    "mermaid": "^11.11.0"
+  }
+}