Clarify that Web Workers only work in Chrome

[ebeloded]

Description

The statement describing Web Workers support was implying that in production mode Web Workers work in browsers other than Chrome. This is not the case: using import statement in worker code would fail production build in Safari and Firefox (reproduction: https://github.com/ebeloded/vite-worker-issue)

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]

I think it's solved in #2494, please try out our latest beta releases.

[ebeloded]

Thanks for the update @antfu. Confirm that the issue is resolved with vite@2.4.0-beta.2 🎉

Hope classic workers support is in the pipeline