Added Vector2DistanceAngle function

[RobinsAviary]

This one is pretty tiny, but it's a function that I always missed from GameMaker, now bundled into raylib's wonderful Vector2 type! I find myself reaching for this calculation pretty often when rendering graphics, so I figured it might be useful to have in the base library.

I set it up to be as easy to use as possible, so it accepts radians for angle. I also applied an offset of -(.5f * PI) to make the function output a line pointing straight up if the user passes in 0.0f for angle. If this is too much, please feel free to let me know, and I can remove it.

Thank you again for the awesome library!

[RobinsAviary]

Oh, also, I can probably make an example for this for the collection if you'd think it'd be useful.

[RobinsAviary]

I see now there is actually a recent example that demonstrates this concept, this function would allow that example to be simplified, if that's something you're interested in.

[raysan5]

@RobinsAviary I prefer to minimize function addition if same result could be accomplished with existing ones, but thanks anyway.