Summary:
@public
Adds `_pt` and `_percent` user defined literals to create `YGValue` instances.
This allows to create `YGValue`s in the following form:
```
use namespace facebook::yoga::literals;
auto a = 123_pt; // == YGValue{123.0f, YGUnitPoint}
auto b = -12.5_percent; // == YGValue{-12.5f, YGUnitPercent}
```
Reviewed By: SidharthGuglani
Differential Revision: D13942100
fbshipit-source-id: ce1e2f9431c3e2a99c6e11896a712539cc535e0d
