Run size limit checks against basics example

[delucis]

What kind of changes does this PR include?

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Description

Our size-limit CI currently checks the docs/dist/_astro/ output for total JS & CSS size, but this makes it tricky for us to add things like #470 as one-off components can breach our budget. As we only really want to ensure a small bundle baseline size, it makes more sense to test against our example projects than docs.

This PR switches across to testing the basics example — unlocked just in time by withastro/astro#7983

(I think this PR will like fail in CI because it tries to run the same script on the PR branch and on main, but the required script isn’t on main yet.)

For the record a comparison of a recent run for docs on main currently and the new results running against the basics example:

	docs	basics
/index.html	13.49 kB	9.59 kB
/_astro/*.js	16.94 kB	16.23 kB
/_astro/*.css	8.75 kB	8.56 kB

As expected, the basics is smaller, but only slightly apart from the home page which has less content and no language switcher markup.

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