feat: allow for async parsing in file loader

[patrickarlt]

Changes

• See Allow file loaders parser option to be async roadmap#1262 for some background.
• Allow the parser() option on the file() loader to return a Promise so that more complex processing can be done if needed.

Testing

See included unit test.

Docs

/cc @withastro/maintainers-docs for feedback!

I'm happy to do a matching PR to the docs if preferred.

[changeset-bot]

🦋 Changeset detected

Latest commit: c15f8b1

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

CodSpeed Performance Report

Merging #14932 will not alter performance

_{Comparing patrickarlt:async-file-parse (c15f8b1) with main (aa58d05)¹}

Summary

✅ 6 untouched

Footnotes

1. No successful run was found on main (33333e8) during the generation of this report, so aa58d05 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩