feat: support for custom commands

[pocketken]

As mentioned in #7, it'd be nice to extend the shell parser to be able to handle additional commands as though they were built-ins/in the system path. Here's a quick take at implementing support by means of extending the CommandBuilder with a handle() method for registering a custom command handler/callback. This should fit in nicely with the custom builder pattern outlined in the documentation.

PR is not so much WIP as at a point where I could use some feedback:

• does this fit with the direction you want things to go? I tried to be somewhat "non-invasive" in terms of how I implemented this, but as its a new project you may have already had ideas on how you wanted to implement this sort of thing down the road.
• not sure I like handle, but command() is obviously taken and customCommand() didn't feel right either.
• if you've got a few commands, one at a time would be tedious. Maybe just take a Record<string, CustomCommandHandler>?

[dsherret]

Yeah, I like this and good idea! I think we could make a couple changes though.

[pocketken]

@dsherret hmm; I am getting different behavior when I run this locally in Windows. Both the original and modified version pass here, which now that I think about it noThrow() would in fact, not throw...

I need another coffee.

[dsherret]

It's probably finding a cd command on mac (via resolveCommand)

[pocketken]

It's probably finding a cd command on mac (via resolveCommand)

Of course it is! ..fires up mac..

$ which cd
/usr/bin/cd
$ which echo
/bin/echo
$ which export
$

export wins! I'll update.

[dsherret]

This all looks very good to me. Thanks a lot! I'm just going to make a few small changes to it myself then merge the PR and do a release.

[pocketken]

👍 no problem, glad to contribute back where I can.

Thanks again for throwing this together!

[dsherret]

It would be nice to build in true and false by the way :)

[pocketken]

I knew you were going to say that :)

I have a few more tasks yet to work through, so, I imagine there will be a PR or two coming with more commands. I'll put those on my list :)

[dsherret]

Thanks again for this and all your other PRs! I released version 0.8.0.