generate __data.json files as siblings of .html files

[Rich-Harris]

This is a possible solution to #8676. It's technically breaking insofar as it clobbers any routes that happen to end with .html__data.json, but given how unlikely that is it's not really breaking.

It solves the problem (that you can't prerender both foo.html and foo.html/__data.json) by making the files siblings — the data is generated as foo.html__data.json instead. This requires no configuration changes, and very little complexity in terms of implementation — no 'is it prerendered?' or 'what do we do with cookies?' questions, just a simple 'does the route end with .html?' check.

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[dummdidumm]

Makes sense! Although it opens up an edge case bug where cookies doesn't match when it shouldn't. If I'm reading this right then setting a cookie path to /docs.html then it would match /docs.html but not /docs.html__data.json (but it would've previously matched /docs/__data.json). Now I'm not sure if this is really relevant in practise, but it's worth thinking about.

Btw I'm very glad we have this behind helper functions, makes this change really easy

[Rich-Harris]

That's true - perhaps the thing to do is duplicate the cookie in the edge case where a) the route ends with .html and b) the cookie path is identical to the request path