Add basic-server-vanillajs example

Demonstrates MCP App SDK usage with vanilla JavaScript (no framework),
providing parity with the existing basic-server-react example.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package-lock.json

@@ -92,7 +92,6 @@
     },
     "examples/basic-server-vanillajs": {
       "version": "1.0.0",
-      "extraneous": true,
       "dependencies": {
         "@modelcontextprotocol/ext-apps": "../..",
         "@modelcontextprotocol/sdk": "^1.22.0",
@@ -1116,6 +1115,10 @@
       "resolved": "examples/basic-server-react",
       "link": true
     },
+    "node_modules/basic-server-vanillajs": {
+      "resolved": "examples/basic-server-vanillajs",
+      "link": true
+    },
     "node_modules/binary-extensions": {
       "version": "2.3.0",
       "resolved": "https://registry.npmjs.org/binary-extensions/-/binary-extensions-2.3.0.tgz",