Triangle: include isFrontFacing

[sciecode]

Includes isFrontFacing method to math Triangle.
Can be a useful test for cutting number of iterations on faces related processes

direction.subVectors( target, camera.position );
// ...
if ( triangle.isFrontFacing( direction ) ) process();

live example

[WestLangley]

@mrdoob This seems to me to be a reasonable addition to the library.

[Mugen87]

It would be super-cool if you could also add a unit test here:

https://github.com/mrdoob/three.js/blob/dev/test/unit/src/math/Triangle.tests.js

[sciecode]

There's something weird going on after #16588 changes. It's saying there are conflicts when there's not. Just gonna close this PR and make a new one.