Find the full path of a shell command.

On POSIX platforms, the PATHEXT environment variable will be unset, so the first command named ‘cmd’ will be returned.

On Windows platforms, the PATHEXT environment variable will contain a semicolon delimited list of executable file extensions, so the first command with a matching path extension will be returned.

@param cmd [String] The name of the command to search for @return [String, nil] The path to the file if found, nil otherwise