This adds reading and writing of USD mesh lights. A mesh becomes a mesh light in USD by attaching the MeshLightAPI to it. USD itself makes reading mesh lights easy by taking on the addition of the relevant attributes when a mesh has a MeshLightAPI attached to it.
Writing mesh lights is also pretty straightforward - we need to intercept a light with a MeshLight shader, attach the MeshLightAPI to the prim and continue on with writing the parameters.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.