fix: respect mainFields when resolving browser/module field (fixes #8659)

[sapphi-red]

Description

Because resolve.mainFields's default value does not include 'browser', I added a special !browser value to avoid a breaking change.
https://vitejs.dev/config/shared-options.html#resolve-mainfields
In Vite 4, I think we could change the default value to ['browser', 'module', 'jsnext:main', 'jsnext'].

fixes #8659

Additional context

---

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.

[patak-dev]

I would prefer to directly wait for Vite 4 and fix this instead of adding the temporal !browser option, but if @sheremet-va @antfu thinks fixing this is important enough for Vitest, I think you did a good call here Sapphi.

[antfu]

I think indeed the current behavior is a kinda flaw and the fix makes a lot of sense. As for the shipping, I think we could have the workaround (maybe via an option instead of a magic !browser?), but I'll leave it to @sheremet-va to decide.

[sheremet-va]

I think indeed the current behavior is a kinda flaw and the fix makes a lot of sense. As for the shipping, I think we could have the workaround (maybe via an option instead of a magic !browser?), but I'll leave it to @sheremet-va to decide.

I was also thinking about some resolve.browserField option or something similar. !browser feels magicky.

JFI: there is also #9860 PR that fixes similar problem.

[patak-dev]

Thanks for the feedback. @sapphi-red what do you think about renaming to resolve.browserField and release it in 3.2?

[sapphi-red]

Yeah, sounds good to me. I've update the PR 👍