feat: enable generatedCode: 'es2015' for rollup build

[antfu]

Description

See https://twitter.com/lukastaegert/status/1440543481059938305

Additional context

Awaits #5045

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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.

[ckken]

support es5?

[morlay]

support es5?

It could. for old browser, we could use AMD/requirejs #5754
So for me, I already say farewell to Webpack.

[Shinigami92]

• Still relevant?
• Should it get out of Draft?
• Do we need tests?
• Clarify why this is not part of plugin-legacy? Do I misunderstood something?

[patak-dev]

@antfu we could merge this one, no? Tests are passing, was there an issue that prevented us from using it? I can't recall now.

[antfu]

We could merge it as long as the tests are passing I believe (it was failing for some reason)

[sapphi-red]

I'm not sure about the failing test in the past, but I have pushed a commit which fixes the current failing test.

[antfu]

Thanks. And it seems like I can't approve my own PR 😅