Shallow routing

[Rich-Harris]

Closes #2673. This is a continuation of #9847, on top of the version-2 branch. Copying the TODOs from that PR:

• Add App.PageState and other types
• remove goto(url, { state })
• intercept history.pushState and history.replaceState to discourage their use
• invalidateAll() should probably use $page.url.href rather than location.href, since those two things can now be different
• fix bugginess around reloads. If you push state and refresh the page, you go to pushed URL (as intended), and hitting the back button will take you to the page you came from, but only because there's special handling for that case. If you push state and click from there to a new page, then reload that page, clicking back twice won't work as expected. Still looking into how to fix this. possibly nuke navigation index history on page reload? unless there's a better way actually i'm not sure what this referred to
• docs
• tests

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