Better bash-less Windows support in actions.run_shell: add command_bat, command_ps

### Description of the feature request:

For better bash-less Windows support,`ctx.actions.run_shell` could add `command_bat` (with the same behavior as genrule's `cmd_bat`) and `command_ps` (with the same behavior as genrule's `cmd_ps`).

This would, in particular, considerably simplify skylib's Windows support, where currently we are forced to write and execute temporary .bat files on Windows (and attempt to deal with the ancient terror of argument quoting in .bat interpreter context - take a look at https://stackoverflow.com/questions/4094699/how-does-the-windows-command-interpreter-cmd-exe-parse-scripts if you don't mind losing some points of sanity).

Pinging potentially interested parties: @brandjon @meteorcloudy @alexeagle

Begin at ctx.actions.run_shell and compare its behavior with genrule's cmd_bat and cmd_ps. Check how skylib currently creates temporary .bat files for Windows support, then define completion as equivalent command_bat and command_ps behavior that removes that workaround.

Written by the indexing model from the issue text.