Extend Base.rationalize instead of defining new function (#56793)

#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.

Author: sostock

base/mathconstants.jl

@@ -29,7 +29,7 @@ end
 Base.@assume_effects :foldable function (::Type{T})(x::_KnownIrrational, r::RoundingMode) where {T<:Union{Float32,Float64}}
     Base._irrational_to_float(T, x, r)
 end
-Base.@assume_effects :foldable function rationalize(::Type{T}, x::_KnownIrrational; tol::Real=0) where {T<:Integer}
+Base.@assume_effects :foldable function Base.rationalize(::Type{T}, x::_KnownIrrational; tol::Real=0) where {T<:Integer}
     Base._rationalize_irrational(T, x, tol)
 end
 Base.@assume_effects :foldable function Base.lessrational(rx::Rational, x::_KnownIrrational)