Constify math types in `bevy_math::rects`

[ickshonpe]

What problem does this solve or what need does it fill?

Tried to use a Rect method in a const function but couldn't. But should be able to.

What solution would you like?

• Constify Rect
  PR: Constify Rect #21905
• Constify IRect
• Constify URect

I already completed Rect (#21905), so someone could copy those changes if they want to pick this up.
I'll finish this up eventually, if no one else wants to.

What alternative(s) have you considered?

Don't do it instead.