[fela-plugin-multiple-selectors] Merge base styles

[robinweser]

Description

This PR fixes a bug dding support for merging base styles with resolved multi-selectors.

Packages

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• fela-plugin-multiple-selectors

Versioning

Patch

Checklist

Quality Assurance

You can also run yarn run check to run all 4 commands at once.

• The code was formatted using Prettier (yarn run format)
• The code has no linting errors (yarn run lint)
• All tests are passing (yarn run test)
• There are no flow-type errors (yarn run flow)

Changes

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• Tests have been added/updated
• Documentation has been added/updated
• My changes have proper flow-types

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