fix: load the jest-dom ESM setup file on Vitest before 1.0

[lxsmnsyc]

Fixes #141 on main (1.x). next requires Vite 8, which Vitest before 1.0 cannot run, so it is not affected.

With Vitest before 1.0, the jest-dom setup file the plugin injects now loads. Before, tests failed with Cannot read properties of undefined (reading 'extend').

Cause

• Vitest before 1.0 resolves setupFiles with require.resolve. That picks jest-dom's CommonJS dist/vitest.js, where require('vitest').expect is undefined.
• The same crash happens without this plugin. The plugin triggers it because it injects @testing-library/jest-dom/vitest automatically.
• Projects on Vite 3 and 4 cannot use Vitest 1.x, so they all hit it.

Changes

• With Vitest before 1.0, the plugin injects the absolute path of jest-dom's ESM entry.
• Vitest 1.0 and later keep the bare specifier.

Testing

• Reproduced with Vitest 0.34.6 on Vite 4.5.14 and 5.0.12, with jest-dom 6.2.0 and 6.9.1.
• With the fix those pass. Vitest 1.0.0, 1.2.1, 4.0.18 and 5.0.5 still pass.
• Added a regression test that fails without the fix.
• vitest run and tsc --noEmit pass.

Note on the first commit

main had no unit tests. The first commit adds the same Vitest setup that next uses: a root vitest.config.ts, a test:unit script, and a step in the Vitest workflow. The other main fix PRs from this batch (#112, #153, #206, #130, #141) share this exact commit, so they do not conflict on it.

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Latest commit: a7ec25f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
vite-plugin-solid	Patch

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