Introduce commands / binaries: open and xdg-open

This will help to open files in default Windows apps for that file extension. Similar commands exist in Mac (open.exe) and in Ubuntu (xdg-open.exe) and are useful to open files in OS's default image viewers or pdf viewers

It can probably be achieved by `alias open="cmd.exe /c"`, but it would be nice if a recommended/tested way existed

The issue names no files or tests; start by locating how WSL exposes commands and how `cmd.exe /c` is invoked. Compare the requested `open` and `xdg-open` behavior with the proposed alias. Done means a tested, recommended way to open files with the default Windows application for their extensions.

Written by the indexing model from the issue text.