Core: Convert Math class to namespace

[Repiteo]

This adopts the formatting seen in godot-cpp which opts for Math as a namespace rather than a class. Given the class already shouldn't be instanced, nor did it really utilize any class-specific functionality, functioning as a namespace feels more apt. Beyond the conversion, the only adjustments were:

• core/math/random_pcg.h include migrated to math_funcs.cpp; explicitly included in the handful of files utilizing RandomPCG directly.
• default_rand, the one private field in the entire class, migrated to math_funcs.cpp as a static variable.
• RANDOM_32BIT_MAX replaced by existing, equivalent define: UINT32_MAX.
• static declaration removed from all functions.
• absd, absf, is_equal_approx, and is_zero_approx moved up—a consequence of needing to be declared before use in a namespace context.
• math_funcs.{h|cpp} functions that weren't using p_* argument syntax now do.
• Math:: prefix removed in header where redundant.
• double sorted before float equivalent where applicable.

[Mickeon]

I assume this change qualifies as codestyle too? Or perhaps I'm missing something.

[Repiteo]

Now that you mention it, yeah; this would technically qualify as that. Given that's already the case, might as well fixup stuff like argument name syntax

[fire]

I want to add my two cents that although this causes churn in the code, it's probably worth doing for consistency in Godot Engine.

The pull request passes the GitHub integration tests, so it's more correct.