Add missing File::PATH_SEPARATOR constant

[postmodern]

Feature Request

It would be useful to have an OS aware File::PATH_SEPARATOR constant. Unlike the regular File::SEPARATOR constant, this constant represents the character used to separate directories in the $PATH environment variable. On Windows the path separator is ';', where as on *nix systems it is ':'.

Example Code

  PATH_SEPARATOR = {% if flag?(:win32) %}
                     ';'
                   {% else %}
                     ':'
                   {% end %}

  PATH_SEPARATOR_STRING = {% if flag?(:win32) %}
                            ";"
                          {% else %}
                            ":"
                          {% end %}

Usage

paths = ENV.fetch("PATH", "").split(File::SEPARATOR, remove_empty: true)

[straight-shoota]

This constant already exists: Process::PATH_DELIITER

There might be a point that it's not very discoverable in the Process namespace. So we could consider moving it somewhere else.
I think the Path namespace would be a better choice than File, though.

[oprypin]

I think Process is the best namespace for it. These aren't actually part of a path in any way and they are needed only to be able to pass this information across processes.
And even if it isn't the best location, the difference isn't enough to justify a move with a deprecation.

[postmodern]

It could be put under ENV, since ENV is actually a module and not a constant Hash.