Add RawFD constructors as an alternative to fd

[LilithHafner]

This provides an alternative to fd for getting the raw file descriptor of a File or IOStream. This is important because fd returns an Int which is a poor choice of return type in the presence of Base.RawFD. I copped most of the docstring and implementation from fd.

This allows folks to write RawFD(::File) instead of RawFD(fd(::File)::Int), and eventually may enable us to deprecate fd.

See also: #51710

[fingolfin]

Looks sensible to me. Would it make sense to backport this to 1.11 (then the docstring of course needs to be adjusted)

[LilithHafner]

fd no longer returns Int since #55080, making this less necessary, though still possibly a good idea.

[KristofferC]

error during bootstrap:
LoadError("sysimg.jl", 3, LoadError("Base.jl", 408, LoadError("libc.jl", 38, UndefVarError(:IOStream, Base.Libc))))
ijl_undefined_var_error at /cache/build/tester-amdci4-15/julialang/julia-master/src/rtutils.c:152
jl_eval_globalref at /cache/build/tester-amdci4-15/julialang/julia-master/src/interpreter.c:159 [inlined]
eval_value at /cache/build/tester-amdci4-15/julialang/julia-master/src/interpreter.c:202
eval_stmt_value at /cache/build/tester-amdci4-15/julialang/julia-master/src/interpreter.c:175 [inlined]

[DilumAluthge]

Removing the "merge me" label because the CI failures seem like they may be related to the contents of the PR.

[LilithHafner]

@whomever, feel free to re-open if you want to see this through. I'm going to close because I no longer think it's necessary.