chore: Add more runtimes/versions to CI

[LekoArts]

Description

In preparation for #22 I wanted to have better integration testing. So this PR adds the following things:

• Run unit tests both with Node 18 and Node 20
• Run integration tests with npm, pnpm, yarn, and bun
• Assert that secco sees correct pm in destination package.json
• Remodel underlying test setup so that adding a new fixture for workspaces will be easier

Checklist

• pnpm run test runs as expected.
• pnpm run build runs as expected.
• (If applicable) Documentation has been updated

[changeset-bot]

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Latest commit: 15f9320

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

Size Change: 0 B 🆕

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