Keep keyframe decimals for prefixed @keyframes

[AdrianBannister]

Proposed changes

This change updates the logic in the evaluateRule function to also look for prefixed keyframes.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• Lint and unit tests pass locally with my changes
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[AdrianBannister]

I noticed there is an issue for this as well: #747

[Ffloriel]

Thanks!