feat(legacy)!: bump modern target to support async generator

[sapphi-red]

Description

After this PR, browsers that doesn't support async generators now runs on legacy chunks.

Plugin-legacy fails to build when the source code included async generators. The reason why this happens is because esbuild doesn't support transpiling async generators. (#11835)

Async generators is supported by edge79, firefox55, chrome63, safari12. The modern target we use currently is es2020, edge79, firefox67, chrome64, safari11.1.
If we changed safari11.1 to safari12, we can avoid transpiling on modern chunks and the build will pass.

Because Safari 11.1 is only used by 0.01%, I think it's fine to bump the target.

I tested this with Safari 11.1, 12.1, Chrome 62, 63, 64, IE11.

close #11835

Additional context

I fixed a bug that I found during implementing this. 5ac8417

When import.meta is not used in the source code, the code was running twice (once with modern chunks and once with legacy chunks) on Chrome 63.

---

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 PR Title 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]

Awesome work

[Pinpickle]

Thanks for sorting this so quickly @sapphi-red!