feat(vercel): allow defining ISR exclusions with regular expressions

[slawekkolodziej]

Changes

There are 2 main changes in this PR:

1. It is now possible to define ISR exclusions as regular expressions.
2. Routes excluded from ISR do not get _isr route entry in .vercel/output/config.json.

Longer answer with my reasoning:

I am using Astro at www.superside.com and our website has many different API routes. I found it quite inconvenient to add every single one of them to the isr.exclude list manually. I prefer to have this in my astro.config.mjs instead:

isr: {
  exclude: [/^\/api/]
}

We've been using this change via patched node module on production for quite some time without any issue. However, this complicates the process of upgrading Astro for us. Ideally I would love to see this change merged into the source code.

As for the second change: as I was working on a way to make isr.exclude accept regular expressions, I noticed that generated vercel/output/config.json file has a lot of unnecessary entries in it (our website has a many excluded api routes). This change helped me investigate Vercel build outputs.

Testing

I adjusted existing tests to handle new use cases.

Docs

/cc @withastro/maintainers-docs for feedback!

If you accept this PR it would be necessary to mention support for regular expressions support in this section of the docs:
https://docs.astro.build/en/guides/integrations-guide/vercel/#excluding-paths-from-caching

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