ci: run vitest on jsdom and happy-dom

[mcous]

Overview

This is the third PR in a series to add multiple Svelte versions and test runners to our CI test matrix.

1. feat(vitest): add vitest cleanup export #294
2. ci: test against svelte v3 and v4 #295
3. ci: run vitest on jsdom and happy-dom #297 <-- you are here
4. TODO add Jest to CI matrix

This PR supersedes or unblocks #286, depending on what we want our default DOM environment to be for local development

Change log

• Add scripts to explicitly test using Vitest + jsdom and Vitest + happy-dom
• Hook those scripts up to the CI matrix

Review requests

How does a CI matrix feel vs running runners on one VM? The matrix keeps the Vitest config simpler, because we don't need to do stuff like isolate the coverage directories, but it uses more "computers" to do so

[github-actions]

🎉 This PR is included in version 4.2.0 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

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