feat: read rgba color values into hex

[jansule]

This reads rgba color values in mapbox styles into hex color values in geostyler style (as expected by geostyler style).

partially solves https://github.com/geostyler/geostyler-mapbox-parser/issues/316

[jansule]

:tada: This PR is included in version 6.0.0-next.1 :tada:

The release is available on:

• npm package (@next dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[jansule]

:tada: This PR is included in version 6.0.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: