TL;DR: Add HasAddSubMul protocol and update Lerp functions to return Vec* where appropriate

Changes

• Add HasAddSubMul protocol type + L TypeVar in arcade.math
• Annotate arcade.math.lerp cleanly to allow arbitarary lerpables
• Make lerp_2d return Vec2
• Make lerp_3d return Vec3
• Docstring updates
• Cross-references for lerp_angle

Follow-up work

• Should we have lerp check for an object.lerp method?
• How do we integrate that into type annotations?

One option for the latter is something like this:

# Assume HasAddSubMul is the same as now

class HasLerp(Protocol[_T, _R]):
    def lerp(other: _T) -> _R:

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