Switch arcade.math.lerp to use a Protocol bound to a TypeVar

[pushfoo]

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:

• Weigh upstreaming the positional slash for maximum fidelity to math built-ins?

[pushfoo]

As an update to this, it looks like only pyglet vector objects have the lerp method. The matrices do not.