Arbitrary margin value proof of concept

[daronspence]

Did I provide a test: YES
Do the tests pass: YES
Is this ready to merge: NO

I used what Adam did in #99 as inspiration for this PR.

I have updated the tests to include a proposed acceptance, though there is still more work to be done to add this support to the other utility classes. The test is definitely brittle.

Since this swaps out asLength for asValue, I'm guessing that might break something else in the framework, though the tests in this repo did not break. ¯\_(ツ)_/¯

I couldn't wrap my head around most of the source code, but I noticed in the asValue() helper function, that there is some special stuff going on for calc(). I wonder if all of the CSS functions could be supported using a similar method?

I also wonder if you can use asLength first, and if that returns undefined to then instead try and look for a supported list of arbitrary CSS functions? I know #102 mentions using var() and my example is clamp()

As a slight tangent, I think this would be super valuable to include, because of the utility of these arbitrary function values. Functions like clamp/min/max etc are extremely helpful for building custom one off design elements.

Great work on the JIT; Overall, it's awesome 🤘

[adamwathan]

Looks good, thanks @daronspence!

[adamwathan]

Ah, just noticed you only updated mt (margin-top), I'll update the rest :)