feat: add 'system' library format

[tristanlins]

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.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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