Refactor navigate

[Rich-Harris]

While working on #9605, I realised that the update(...) function in client.js is really quite badly designed. It's used for both navigations and invalidations, but it spends so much time deciding which of those things it's dealing with that it really shouldn't be a single function.

In fact, in the invalidation case almost none of the logic is actually used. We can move that stuff into the invalidate function, and inline the entirety of update into navigate. This means less indirection, and easier-to-follow code.

This change will make it easier to add a complete promise to the Navigation interface.

No tests or changeset necessary.

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[dummdidumm]

👍