Semantics of `seek`

[jakobnissen]

I'm not sure what the precise semantics of seek should be. Currently, it's underdefined, and I think we should exploit that the current lack of documentation (and inconsistent behaviour) gives us freedom to define seek as we want (rather than just documenting how it actually behaves currently).

• Is the position always an integer, or can an IO define a custom indexing type e.g. to carry state?
• Should it be documented that the position must be zero-indexed?
• What should the behaviour be when seeking to a negative offset? This errors for IOStream, but not for IOBuffer
• What should the behaviour be when seeking to something larger than the filesize? Currently, it seeks to the end, but @vtjnash in Refactor IOBuffer code #57570 suggested it should instead behave like lseek.
• What should happen when calling seek on an unseekable IO? Should it be a noop or throw an error? Which error?

[nhz2]

It would be nice to have a function that acts like pwritev2 for use in ZipArchives.jl. The current code I have that uses seek and position is messy and probably not as performant as it could be: https://github.com/JuliaIO/ZipArchives.jl/blob/99b5e8d99b048de340f2cc3c2b1f807b8f2c8789/src/writer.jl#L344-L359