Add space-evenly property value for justify-content and align-content

[TriangulumDesire]

This is a small change, adding support for space-evenly to the flex-box properties justify-content and align-content. It is similar to space-around, but the gaps between the items and the container edges are equal to the gaps between the items (instead of the gaps between the edges and items being half the size of the space of the gaps between items).

I couldn't find any unit tests that explicitly check all of the flex-box spacings, but I did test it visually, and it works nicely.

space-around:

space-evenly:

[mikke89]

Nice one, thank you!

It would be very helpful if you could add these values to the documentation as well.

We normally use the visual tests to test things like this. I ran this PR through all the flex tests, including the W3C ones, and it didn't really make any difference. So at least we didn't break anything but it would be nice with better test coverage.