Perf improvement for 2 arg show for `UnitRange` type (#57823)
Mentioned in
https://github.com/JuliaLang/julia/pull/57697#discussion_r2001497340 and
https://github.com/JuliaLang/julia/pull/57697#discussion_r2001723774
also improves #57697
benchmarks on my configuration:
```julia
julia> f(io, a) = print(io, repr(first(a)), ':', repr(last(a)))
f (generic function with 1 method)
julia> function g(io, a)
show(io, first(a))
print(io, ':')
show(io, last(a))
end
g (generic function with 1 method)
julia> using BenchmarkTools
julia> @btime f(io, a) setup=(io=devnull; a=big(2)^(2^10):big(6)^(2^10);)
4.889 μs (17 allocations: 5.23 KiB)
julia> @btime g(io, a) setup=(io=devnull; a=big(2)^(2^10):big(6)^(2^10);)
4.021 μs (11 allocations: 3.86 KiB)
julia> @btime f(io, a) setup=(io=devnull; a=10000000000000000:10000000000000001;)
312.814 ns (12 allocations: 576 bytes)
julia> @btime g(io, a) setup=(io=devnull; a=10000000000000000:10000000000000001;)
62.859 ns (4 allocations: 208 bytes)
```
