Add os::split_paths

Adds a platform-specific function, `split_paths` to the `os` module. This
function can be used to parse PATH-like environment variables according to
local platform conventions.

Closes #14352.
auto merge of #14544 : aturon/rust/issue-14352, r=alexcrichton

Adds a platform-specific function, `split_paths` to the `os` module. This
function can be used to parse PATH-like environment variables according to
local platform conventions.

Closes #14352.