fix: return plaintext 404 for anything under appDir

[Rich-Harris]

closes #9802. Any requests to /_app/* (other than /_app/env.js) that make it to the SvelteKit server (i.e. they're not already handled by a static webserver) immediately result in a plaintext 404, instead of doing anything more involved.

Today:

• /missing
• /_app/missing

With this PR:

• /missing (same)
• /_app/missing (plaintext 404)

@Conduitry since this was your issue, thoughts on a) whether this is technically a breaking change and b) whether this plaintext response suffices or we should use src/error.html?

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[elliott-with-the-longest-name-on-github]

LGTM, though I would prefer the error template, to at least give users some control over what this returns.

[Rich-Harris]

yeah, that's reasonable. then again we issue plaintext responses for reroute errors, malformed URIs, paths above base and some other cases, so we should probably keep it like this for consistency and then decide if we want to change all of them