chore: converted PhysicsMaterial2D's to classes and renamed Default to be ReadOnly

Engine.Physics2D/ReadOnlyPhysicsMaterial2D.cs

@@ -0,0 +1,9 @@
+namespace Engine.Physics2D;
+
+public readonly struct ReadOnlyPhysicsMaterial2D(float Friction, float Restitution) : IPhysicsMaterial2D
+{
+    public readonly float Friction { get; } = Friction;
+    public readonly float Restitution { get; } = Restitution;
+
+    public readonly static ReadOnlyPhysicsMaterial2D Default = new(.1f, .1f);
+}