PLYExporter: Add support for custom properties

[Danilkat]

Description

Right now, PLYExporter supports only those attributes: vertex positions, colors, normals and uv coordinates. So this means that exported mesh wont have other attributes.

Solution

Add support for custom attributes export in PLYExporter. The logic might be similiar to customPropertyNameMapping in PLYLoader.

Alternatives

None

Additional context

Its strange to have this support in PLYLoader but not in PLYExporter. There might be some caveats with the objects that have multiple children and geometry with different custom attributes.

[mrdoob]

Seems like you've isolated what needs to be changed. Would you like to do a PR with it?

[Danilkat]

Yes, thats what im about to do in the following days

[Danilkat]

Update:
Ive rewritten this class in my environment, but in TS. I will make a PR when ill port it to JS.