Add dependencies to aliases & functions

[themattman]

If an alias or function requires a program, like jira-cli, declare this when registering the item.

Functions

must be explicit about dependent programs

Alias Detection

Perhaps run the base program in an alias and detect this?

Rough Proposal

_add_function jira-cli-open jira # `jira` is the program

When sourcing, warn and drop these functions if the dependency doesn't: exist in PATH? not executable? returns non-zero on --version or similar?