Add shorthand for Starlight templates to create astro

[delucis]

Changes

This adds some more special case handling for Starlight starter templates

Currently we hardcode --template starlight to withastro/starlight/examples/basics.

But withastro/starlight#337 adds a new starter, and it would be nice to shorten the command needed to use it.

# current syntax
npm create astro@latest -- --template withastro/starlight/examples/tailwind

# with the changes in this PR
npm create astro@latest -- --template starlight/tailwind

If we think this is too much special-casing, we can close this.

Testing

Tested the logic works manually, but not sure if it’s possible to run the local create-astro?

Docs

Will only impact Starlight’s docs, which I’ll update if we merge this.

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Latest commit: dd472f2

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[delucis]

Ah yeah, I did consider this but this version does potentially discard stuff if there are two slashes. Don't see when we'd need that though to be fair — thanks for merging!