fix(plugin-legacy): don't include SystemJS in modern polyfills

[Menci]

Description

Currently the legacy plugin bundles SystemJS in both legacy and modern (if any) polyfills if we set options.externalSystemJS to false. But on the modern target it doesn't use SystemJS.

I just pass false when building modern polyfills. The parameter externalSystemJS is renamed to excludeSystemJS to avoid ambiguity.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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