feature: add DefaultCommand field to App

[jalavosus]

See issue #1307 for context.

What type of PR is this?

• feature

What this PR does / why we need it:

Enables a CLI application to run a default Command (by name) if no command name is passed as a cli argument.

Which issue(s) this PR fixes:

Fixes #1307

Special notes for your reviewer:

Genuinely unsure if I implemented this correctly, but the test cases look sane on my end and they pass.

Release Notes

cli.App now has a `DefaultCommand` field, which, if set, enables a cli.App to run a specific command if no command is otherwise passed as an argument. 

[jalavosus]

Just had the thought that it might make sense to have the flag name check also check for default command's subcommands (should they exist), to make the whole thing real seamless.

[dearchap]

I dont see a real use case for this. Are there any real world clis which make use of this functionality.

[jalavosus]

I dont see a real use case for this. Are there any real world clis which make use of this functionality.

@dearchap

• https://github.com/ahmetb/kubectx (defaults to listing all configured kubectl contexts)
• yarn (defaults to yarn install)
• python/python3 (defaults to loading a REPL if no arguments are specified)

Also, if the feature is able to be implemented (which, well, it clearly is) then why not implement it? It adds functional value to the package, and doesn't seem to be -- at least imo -- any sort of feature creep.

[meatballhat]

@jalavosus Hello and sorry for the delay 😭 ! Are you up for resolving conflicts and getting tests happy? I agree with you regarding examples in the wild and I'd like to move forward with this work 👍🏼

[jalavosus]

@meatballhat Rebased onto main (hence the force push) and fixed the failing test (just needed to run gofmt)

[meatballhat]

@jalavosus Thank you! Are you able to appease codecov, too? 😅

[jalavosus]

Are you able to appease codecov, too?

I've been trying to, it does not like breaks.

[meatballhat]

Lovely! 🤩