chore: generate coverage report when deploying site

[Rich-Harris]

This outputs a coverage report to sites/svelte-5-preview/static/coverage, allowing us to easily view the coverage report. (A more complete solution would report regressions in coverage, but that's a level of faffing I don't have the energy for.)

Perhaps more excitingly, this allows us to remove --coverage from the default test script, which makes tests run much faster. I should have done this forever ago, sorry

Sample coverage report (linked from /status)

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Latest commit: 1c1904f

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