Add support for BrowserType.executablePath(), to return the path that xk6-browser expects to find browser executable.

Relevant links:

• Playwright docs: https://playwright.dev/docs/api/class-browsertype#browser-type-executable-path
• xk6-browser code starting point: https://github.com/grafana/xk6-browser/blob/main/chromium/browser_type.go#L61
• Playwright code: https://github.com/microsoft/playwright/blob/master/packages/playwright-core/src/server/browserType.ts#L47
• Playwright tests: https://github.com/microsoft/playwright/blob/master/tests/browsertype-basic.spec.ts