Add support for keyword values for min-width and max-width
This is a follow-up to #14432 which got closed and can no longer be re-opened.
This PR aims to add support for keyword-values max-content, min-content, fit-content, and fill-available to the properties that use them, namely min-width, min-height, max-width, and max-height.
It's still untested because I still haven't figured out how to do that. I guess I should write or find some web page that uses these properties.
Refs #13821.
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Implement an ExtremumLength type which contains all the enumerated
keyword values for min-width, min-height, max-width, and
max-height. Then, implement a MinLength which can be used for min-width
and min-height. So far this just maps to Gecko values.
Refs #13821.
