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