fix: mains by format fallbacks

[rschristian]

Fixes #963

Typo was from https://github.com/developit/microbundle/pull/950

We were missing tests for controlling the output filenames from package.json nearly entirely, both the common fields ("main", "module", etc.) and their alts ("cjs:main", "esmodule", etc.). This adds a test suite for both (though there's still a few untested fields, like nested "exports" and "syntax.esmodule" entry).

[changeset-bot[bot]]

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Latest commit: a624894771419efd18a0f9eda2997f0f648adcd4

The changes in this PR will be included in the next version bump.

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microbundle	Patch

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