feat: add MCP Chrome extension (#325)

Instructions:

1. `git clone https://github.com/mxschmitt/playwright-mcp && git
checkout extension-drafft`
2. `npm ci && npm run build`
3. `chrome://extensions` in your normal Chrome, "load unpacked" and
select the extension folder.
4. `node cli.js --port=4242 --extension` - The URL it prints at the end
you can put into the extension popup.
5. 
Put either this into Claude Desktop (it does not support SSE yet hence
wrapping it or just put the URL into Cursor/VSCode)

```json
{
  "mcpServers": {
    "playwright": {
      "command": "bash",
      "args": [
        "-c",
        "source $HOME/.nvm/nvm.sh && nvm use --silent 22 && npx supergateway --streamableHttp http://127.0.0.1:4242/mcp"
      ]
    }
  }
}
```

Things like `Take a snapshot of my browser.` should now work in your
Prompt Chat.

----

- SSE only for now, since we already have a http server with a port
there
- Upstream "page tests" can be executed over this CDP relay via
https://github.com/microsoft/playwright/pull/36286
- Limitations for now are everything what happens outside of the tab its
session is shared with -> `window.open` / `target=_blank`.

---------

Co-authored-by: Yury Semikhatsky <yurys@chromium.org>

tests/files.spec.ts

@@ -100,7 +100,8 @@ The tool "browser_file_upload" can only be used when there is related modal stat
   }
 });
 
-test('clicking on download link emits download', async ({ startClient, server }, testInfo) => {
+test('clicking on download link emits download', async ({ startClient, server, mcpMode }, testInfo) => {
+  test.fixme(mcpMode === 'extension', 'Downloads are on the Browser CDP domain and not supported with --extension');
   const { client } = await startClient({
     config: { outputDir: testInfo.outputPath('output') },
   });
@@ -124,7 +125,8 @@ test('clicking on download link emits download', async ({ startClient, server },
 - Downloaded file test.txt to ${testInfo.outputPath('output', 'test.txt')}`);
 });
 
-test('navigating to download link emits download', async ({ startClient, server, mcpBrowser }, testInfo) => {
+test('navigating to download link emits download', async ({ startClient, server, mcpBrowser, mcpMode }, testInfo) => {
+  test.fixme(mcpMode === 'extension', 'Downloads are on the Browser CDP domain and not supported with --extension');
   const { client } = await startClient({
     config: { outputDir: testInfo.outputPath('output') },
   });