Absorb the Mink extension glue in-tree

[AlexSkrypnyk]

friends-of-behat/mink-extension pledges no updates beyond Symfony compatibility, and its bundled MinkContext is the second grammar v4 retires. The glue ships in-tree; behat/mink and the browser drivers stay upstream.

• Behat\Mink\ owns the extension: the mink config schema (sessions, base_url, browser_name, files_path), driver factories for browserkit and selenium2, BrowserKitFactory, DocumentElement.
• A RawMinkContext equivalent (roughly 150 lines: getSession(), assertSession(), visitPath(), getMinkParameter()) becomes RawContext's in-tree ancestor - decided at v4.0.0 because the ancestry is public API and cannot swap in a 4.x.
• friends-of-behat/mink-extension moves from require to suggest as the interop seam: an adapter accepts upstream DriverFactory implementations when that package is installed.
• dmore/behat-chrome-extension keeps working through the adapter (the chrome_headless CI legs prove it).