Use custom header to distinguish between +page.server.js and +server.js requests in enhance

[Rich-Harris]

Closes #6983

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[dummdidumm]

Ah wait, should we give this a more generic name, so it signals that this also works to distinguish GET requests? Also, this should probably be mentioned in the docs.

[dummdidumm]

Yeah on second thought, that doesn't make sense, you don't want to do a fetch-get getting the HTML string (and if you do, just do text/html)