Require a separate installation of MSW

[sheremet-va]

Clear and concise description of the problem

@vitest/browser always installs MSW even if module mocking is not used.

Suggested solution

Instead of always installing MSW, maybe we should throw an error on vi.mock and ask for an MSW installation.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[julienmonnard]

Hello!
I was about to file an issue when I saw this one, so I'll add some extra context here 🙂

I think this is a good idea, not only for the reason mentioned in the description above, but also because it seems to conflict with other installs of msw, expecially when these are not the same version.

I'm currently using msw@2.2.14 fixed version, and every time I do an npm install, the mockServiceWorker.js generated by msw will be for the version defined in @vitest/browser (currently msw^2.7.3).

If I keep the one generated for 2.7.3, I will get issues in my tests, and if I keep the one generated for 2.2.14, I will get issues in my mocks 😅

I guess making msw a peer dependency (required or optional) could help solve this conflict?