fix(legacy): fix broken build when renderModernChunks=false & polyfills=false (fix #14324)

[k-yle]

Closes #14324

Description

Issue #14324 has a detailed explaination of the bug. In short, if the legacy plugin is configured with renderModernChunks=false & polyfills=false then the outputted build is broken and unusable.

This PR fixes the bug by including the ployfill chunk in this scenario, since the ployfill chunk contains SystemJS.

Additional context

I'm not sure how to add tests without creating an entirely new folder under /playground, since a test for this PR would require a different vite.config.ts file.

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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 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.

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[sapphi-red]

Thank you!

[k-yle]

I don't think the test failure in playground/assets is related to this PR...