Add <fractal2d> node

[ld-kerley]

Mirroring the existing behavior for <fractal3d>, we add the corresponding <fractal2d> node that uses the texcoord` input as its manifold.

Add support for OSL, MSL, GLSL, MDL (note : MDL not tested).

Updated specification documents to add <fractal2d> to the specification and remove it from the proposals document.

Also remove period inputs for the noises that don't have those inputs in the specification. This is already documented in the proposals document.

Note : in the GLSL noise library code I renamed the existing mx_fractal_noise_xxx() functions to mx_fractal3d_noise_xxx() to make the differentiation cleaner with the new mx_fractal2d_noise_xxx(). But I retained wrapper functions with the old name that just call through to the new name for backwards compatibility. I was unsure how much custom customer code might be referencing those functions. We can remove the shim functions when we have another breaking API release.