Fix: Added keyword support

[oliverfoster]

fixes #77

Fix

• Allow 'initial', 'inherit', 'unset', 'revert' and 'revert-layer' to pass-through modification rules

References

https://developer.mozilla.org/en-US/docs/Web/CSS/computed_value

[oliverfoster]

@StuartNicholls did you manage to review this?

[github-actions[bot]]

:tada: This PR is included in version 2.2.2 :tada:

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