Fix big(1)^big(-1) (#50821)
This PR gives BigInt exponentiation the same edge-case behavior as other
integer exponentiation.
The normal integer exponentiation checks if `x` is `1` or `-1` before
erroring if the power is negative.
https://github.com/JuliaLang/julia/blob/9f9e989f241fad1ae03c3920c20a93d8017a5b8f/base/intfuncs.jl#L283-L287
With this PR `big(1)^big(-1) == 1`, `big(-1)^big(-1) == -1` and
`big(-1)^big(-2) == 1`
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Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>
