allow prefixes and suffixes around rest parameters

[Rich-Harris]

Fixes #1714. Not sure why rest segments were ever required to be standalone — probably just me being too lazy to figure out the implementation and not being able to see a valid use case. Since there clearly is a valid use case, and nothing seems to break if we remove that restriction, I finally got around to figuring out the implementation. (Turns out it's not that hard.)

Prefixes probably aren't all that useful, but suffixes (i.e. /tree/[...rest].json.js) certainly are.

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[benmccann]

I didn't look at the code, but it seems like a fairly safe and desirable change and has tests, so I say ship it!