[Feature] support WebKit Technology Preview

[aslushnikov]

We currently ship with WebKit Tip-Of-Tree. However, we'd like to enable our clients to use the same webkit that is shipped in Safari Technology Preview.

The plan for now:

• introduce a new "technology-preview" webkit channel

  const browser = await playwright.webkit.launch({channel: 'technology-preview'});

• enable CLI to download this channel

  $ npx playwright install webkit-technology-preview

• wire tests to run with this channel as default
• make sure tests are annotated with a proper version (e.g. webkit version + build revision) to differentiate in flakiness dashboard