Transition only between pages where both have ViewTransitions enabled

[martrapp]

Changes

Before:
The history could contain states which are not handled by the client-side router when it dies during a visit to pages without <ViewTransitions>. The client-side router dies when a page w/o <ViewTransitions> is (re)loaded (no swap). The load deletes the router's modules. This is not a problem if the user navigates back to a page with <ViewTransitions> and this is reloaded. But if the browser history contains an entry for that page that was originally created by the router, the browser expects the now-deceased router to force the reload for that history entry. This results in a broken backward navigation.

Change:
With this change, new history entries and (view transitions) will only be created if the target page also defines <ViewTransitions>. This way the browser's backward navigation will always restore the router in time, even if lost on pages without <ViewTransitions>.

Testing

Added e2e test to prove that a killed router is revived correctly.
Adjusted existing tests because navigation to pages without <ViewTransition> leads to reloads.

Docs

n/a. bug fix

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Latest commit: 0fedb9a

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[matthewp]

Looks great! Should note that we checked <meta name="view-transition" content="same-origin" /> and it behaves the same way as this PR. This prevents bugs were you can get "stuck" going forward to pages without VT and then trying to come back.