ExtremumLength is the keyword type for css sizing properties, so we
could use cbindgen.

In Gecko, we use nsStyleCoord to store the sizing properties, and use
integer values to check the enum values, so I keep the macros in nsStyleConsts.
Even though we need to convert the enum type into integer, we still have
benefits to reduce the complexity of converting Rust into C++, and leave
the simplified mappings in C++ for better readability.

Differential Revision: https://phabricator.services.mozilla.com/D7535