Description

Added system to the allowed library formats.
This allows to generate a SystemJS compatible library.

Additional context

SystemJS is a very popular module loader, and it's supported by rollup out of the box. We using SystemJS in our system. Currently I build an ES library with vite and then rebuild to system with rollup.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• 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.