Add `ComplexRotation <: Rotation{2}`

We now have two types for 2D rotations:

• RotMatrix{2} stores four real values as a matrix
• Angle2d stores one real value as a rotation angle

However, a 2D rotation can be parametrized with two real values $(c,s)$ like

$$
R = \begin{pmatrix}c & -s \\ s & c\end{pmatrix}.
$$

This rotation is the same as a rotation by a complex number $c+is$, and can be implemented like this:

struct ComplexRotation{T} <: Rotation{2,T}
    c::Complex{T}
end

This is sometimes useful when we need to generate a rotation from 2D vectors.
See rotation_between for example.
https://github.com/JuliaGeometry/Rotations.jl/blob/f00b07d9d809717bfb6b1d9fc3966e6be059af22/src/rotation_between.jl#L9-L15

Start by reading src/rotation_between.jl at the linked lines, then compare how RotMatrix{2} and Angle2d implement the Rotation{2} interface. The issue is done when ComplexRotation{T} is added as the proposed Rotation{2,T} implementation and can support the 2D-vector rotation use case described by rotation_between.