Support "align-content: space-evenly" (#1422)

Summary:
X-link: https://github.com/facebook/react-native/pull/41019

### Changes made
- Regenerated tests (as some aspect ratio tests seem to be out of date compared to the fixtures)
- Added SpaceEvenly variant to the "Align" enums (via enums.py)
- Implemented `align-content: space-evenly` alignment in CalculateLayout.cpp
- Added generated tests `align-content: space-evenly`
- Updated NumericBitfield test to account for the fact that the Align enum now requires more bits (this bit could do with being reviewed as I am not 100% certain that it's valid to just update the test like this).

### Changes not made
- Any attempt to improve the spec-compliance of content alignment in general (e.g. I think https://github.com/facebook/yoga/pull/1013 probably still needs to happen)

Pull Request resolved: https://github.com/facebook/yoga/pull/1422

Reviewed By: yungsters

Differential Revision: D50305438

Pulled By: NickGerleman

fbshipit-source-id: ef9f6f14220a0db066bc30db8dd690a4a82a0b00

gentest/gentest-javascript.js

@@ -120,6 +120,7 @@ JavascriptEmitter.prototype = Object.create(Emitter.prototype, {
   YGAlignStretch: {value: 'Align.Stretch'},
   YGAlignSpaceBetween: {value: 'Align.SpaceBetween'},
   YGAlignSpaceAround: {value: 'Align.SpaceAround'},
+  YGAlignSpaceEvenly: {value: 'Align.SpaceEvenly'},
   YGAlignBaseline: {value: 'Align.Baseline'},
 
   YGDirectionInherit: {value: 'Direction.Inherit'},