Auto merge of #14740 - glasserc:extremum-length, r=Manishearth

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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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #13821  (github issue number if applicable).

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components/style/values/computed/mod.rs

@@ -23,6 +23,7 @@ pub use super::specified::{Angle, BorderStyle, GridLine, Time, UrlOrNone};
 pub use super::specified::url::{SpecifiedUrl, UrlExtraData};
 pub use self::length::{CalcLengthOrPercentage, Length, LengthOrNumber, LengthOrPercentage, LengthOrPercentageOrAuto};
 pub use self::length::{LengthOrPercentageOrAutoOrContent, LengthOrPercentageOrNone, LengthOrNone};
+pub use self::length::{MaxLength, MinLength};
 pub use self::position::Position;
 
 pub mod basic_shape;