fix(plugin-legacy): fix type errors

[llc1123]

Description

Fixed multiple typecheck errors from TypeScript.

Additional context

plugin-legacy/index.js l180: the return type is incomplete (and having a syntax error).
plugin-legacy/index.js l547: polyfillChunk could be type RollupWatcher which doesn't have the output property.

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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.
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• 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.