This introduces the `*name=body` syntax to capture intermediate result.
For example, `*css=section >> "Title"` will capture a section that contains "Title".
This encapsulates selectors logic in one place, in a preparation for more complex scenarios like main-world selectors or piercing frames.
Note: we had `Page.fill should wait for visible visibilty` test, but we do not actually wait for visible in page.fill(). It happened to pass due to lucky evaluation order.
References #1316.
