Fix styling non-standard pseudo elements

[MattCrouch]

Description

This PR fixes an issue with styling pseudo-elements the browser does not understand. This includes vendor-specific elements such as ::-moz-progress-bar.

The cached scoreIndex value would be increased but insertRule method would not add the rule. Any further rules would then result in an IndexSizeError.

Packages

• fela-dom

Versioning

• Major
• Minor
• Patch

Checklist

Quality Assurance

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• 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

[robinweser]

LGTM