Extend Base.rationalize instead of defining new function Base.MathConstants.rationalize

[sostock]

#55886 accidentally created a new function Base.MathConstants.rationalize instead of extending Base.rationalize, which is the reason why Base.rationalize(Int, π) isn’t constant-folded in Julia 1.10 and 1.11:

julia> @btime rationalize(Int,π);
  1.837 ns (0 allocations: 0 bytes)      # v1.9: constant-folded
  88.416 μs (412 allocations: 15.00 KiB) # v1.10: not constant-folded

This PR fixes that. It should probably be backported to 1.10 and 1.11.