Add math.diag

[olokobayusuf]

It'll be great to be able to do the following:

float2 scale = float2(2f, 3f);
float2x2 transform = diag(scale);
// Yields:
// 2f 0f
// 0f 3f

[unpacklo]

Great idea, I'll be considering it for a future version of Mathematics!

[ltmx]

I implemented it for you my friend, in Unity.mathx :)
Commit => ltmx/Unity.mathx@868d045

[olokobayusuf]

@ltmx thanks for this! I'm surprised this issue has gotten no movement for almost three years now.