Support for `custom-ident` type

[chsxf]

Support for custom-ident type, used for example in the transition or transition-property CSS properties.

https://developer.mozilla.org/en-US/docs/Web/CSS/transition
https://developer.mozilla.org/en-US/docs/Web/CSS/transition-property
https://developer.mozilla.org/en-US/docs/Web/CSS/custom-ident

Strategy: Consider parsing the unquoted string to this type of identifier if no keyword is recognized and the property accepts it.