Add allocation-free vector perpendiculars, 2x2 matrix inverse and solve, and Double.IsFinite

Add four small, generally useful maths capabilities requested by the Physics2D physics-library port and valuable to any 2D maths code:

• Vector2.LeftPerpendicular() and Vector2.RightPerpendicular() — in-place 90-degree rotations, (-y, x) and (y, -x), returning the vector for chaining. These provide allocation-free perpendicular and scalar/vector cross-product forms, complementing the existing allocating Vector2.Cross(scalar, v) / Vector2.Cross(v, scalar) overloads.
• Matrix.Inverse(), Matrix.Inverse(out), and Matrix.InvertSelf() — the 2x2 inverse. A singular matrix (zero determinant) inverts to the zero matrix, matching common 2D-physics conventions.
• Matrix.Solve(b) and Matrix.Solve(b, out) — solve Matrix * x = b for a Vector2 x without forming the inverse. A singular matrix solves to the zero vector.
• Double.IsFinite() — True when the value is neither NaN nor infinity, clearer than Not v.IsNaN() And Not v.IsInfinity().