make write/unsafe_write on iobuffer return signed like other io

[KristofferC]

Noticed by @tkelman in #20608 (comment).

Before:

julia> io = IOBuffer()
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1)

julia> write(io, "SGVsbG8h")
0x0000000000000008

julia> write(Base64EncodePipe(io), "SGVsbG8h")
8

julia> write(open("bla", "w"), "SGVsbG8h")
8

[tkelman]

I wonder what this might break, but I think we usually go for signed ints in most API's like this. x-ref 2af73dd (and the fixup 7535258) where changing it the other way around broke a whole bunch of things

[KristofferC]

Is this breaking and should thus have a NEWS entry?