Cleanup redundant code in App/Command

[dearchap]

What type of PR is this?

(REQUIRED)

• cleanup

What this PR does / why we need it:

(REQUIRED)

Current code path requires almost similar code is three separate places. App.RunContext, App.RunAsSubcommand, and Command.Run. Any bug in flag handling/help etc would need changes in all these places. To make it more manageable the proposal is to condense it into a single place in Command.Run. To enable this we simulate the main App instance as a "rootCommand".

This is a draft proposal. It still requires some work to have all unit tests pass but I wanted some buy in from the community

Which issue(s) this PR fixes:

(REQUIRED)

Fixes #554

Special notes for your reviewer:

(fill-in or delete this section)

Testing

(fill-in or delete this section)

Release Notes

(REQUIRED)

[meatballhat]

fwiw the test-docs check failure seems to be revealing something that's unhappy somewhere

https://github.com/urfave/cli/actions/runs/3215687734/jobs/5256958204#step:7:244

# ---- >8 ----
time="2022-10-09T21:52:33Z" level=info msg=done error_count=1 example_count=29 fields.time=13.723986848s source_count=15
time="2022-10-09T21:52:33Z" level=error msg="expected output does not match actual: \"made it!\\nPhew!\" != \"\""