Allow to call an app with main appbuilder binary

[jay7x]

I hit an issue with appbuiler when using it with mise environment manager (with UBI backend). In this case one should relink their apps with every appbuilder upgrade (because it'll be installed into another directory). Another option is to link an app to a mise "shims" directory, but those shims are already symlinks to mise itself. So it cannot route the call to appbuilder.

So it'd be quite useful to have ability to call an app directly from appbuilder binary. For example:

appbuilder exec <app> [parameter ...]
# - OR -
appbuilder run <app> [parameter ...]