Loosen type restriction on `Ryu.writeshortest` (#57172)
`Ryu.writeshortest` is documented to have a method that "allows passing
in a byte buffer", just like `Ryu.writefixed` and `Ryu.writeexp`, but
unlike those functions `writeshortest` is type constrained to
`::Vector{UInt8}`. This PR loosens that to `::AbstractVector{UInt8}`, to
allow the "byte buffer" to e.g. be a `Memory` rather than a `Vector`.
I've added tests and updated the docstrings for all three functions to
ensure that they're not just restricted to `Vector{UInt8}`.
This change was prompted by our private codebase hitting `MethodError:
no method matching writeshortest(::Memory{UInt8}, ::Int64, ::Float64,
...)` when trying it out with Julia v1.12.0-DEV.
(cc @quinnj -- i think you added this method originally, but i couldn't
see any reason why e.g. Memory shouldn't be allowed now we have it)
