basecommands: sh_help: search or list commands

This sh_help allows users to search for commands. If no search term is specified, all available commands are displayed. Displayed commands are always paginated, with additional pages viewed by executing the same command again.

I thought @Person8880 or @BrightPaul requested this of me somewhere, but I can't find that now. My apologies if this pull request feels unplanned. I prefer to discuss and coordinate before working collaboratively. Discard this code if you wish. Alternatively, advise how you'd like it changed/improved.