feat: add mention of possibility of perhaps async in the file() loader

[Princesseuh]

Description (required)

For withastro/astro#14932. The parser function can now be async, which is pretty neat if you want to fetch files every time you parse your .csv (maybe don't do that)

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[sarah11918]

Looks good, just what we needed! 🫡

[sarah11918]

Oops, forgot that the reference entry for parser() at docs.astro.build/en/reference/content-loader-reference#parser needs updating (both in current docs and in v6 docs) to now show that this can be async.

I think @ArmandPhilippot suggested what this needs to look like in the other PR? withastro/astro#14932 (comment)

[ArmandPhilippot]

Yes, we would need to update the type with:

(text: string) => Record<string, Record<string, unknown>> | Array<Record<string, unknown>> | Promise<Record<string, Record<string, unknown>> | Array<Record<string, unknown>>>

to show in the reference that the parser can be async.