Add <fract> node

[pablode]

As discussed, this PR adds a node that uses the intrinsic of GLSL/MSL/MDL.

(Note: not tested with MDL & OSL, but should match their specifications.)

[jstone-lucasfilm]

This looks great to me, thanks @pablode, and I'm CC'ing @dbsmythe, @anderslanglands, and @ld-kerley for their thoughts.

To address the testing errors, you'd just need to add a single example of each type variant of fract in the Materials/TestSuite folder, following the pattern for modulo seen here:

https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/resources/Materials/TestSuite/stdlib/math/math_operators.mtlx#L325

[ld-kerley]

for other operators we provide node definitions for both the color3/4 and vector2/3/4 data types. Has it already been discussed elsewhere? Is there a reason to not also include versions here for color3/4?

[pablode]

Good observation. I was hesitant to add these definitions since defining math operations (e.g. modulo) on colors felt a bit sketchy. What if color means "wavelengths" in the future? Besides, the values are usually normalized in the [0, 1] range, making the operator less useful.

But yes, I'll add them with the next push.