fix: use Vitest for unit testing, clean regex bug

[antfu]

Description

Currently, we are mixing the integration tests and unit tests. With the goal that we might use Vitest exclusively in the future, this PR takes the progressive approach by letting Vitest handles the unit tests first, providing a better DX over each feature.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[antfu]

Interestingly this exposes a bug of Vite itself.

vite/packages/vite/src/node/plugins/workerImportMetaUrl.ts

Line 115 in 474d5c2

/\bnew\s+(Worker|SharedWorker)\s*\(\s*(new\s+URL\s*\(\s*('[^']+'|"[^"]+"|`[^`]+`)\s*,\s*import\.meta\.url\s*\))/g

This regex breaks our logic of lexing strings

vite/packages/vite/src/node/cleanString.ts

Line 26 in e48827f

;[clean, lastEnd] = lexStringTemplateExpression(res, start)

[bluwy]

Your bug fix may indirectly resolve #8014 too