Standardize command related names

[st0012]

Before we expose command extension APIs, I think it'd be beneficial to first standardize (or even modernize) the names we use for command related concepts. This includes namespace, base class, and folder names:

1. Rename lib/irb/extend-command.rb to lib/irb/command.rb
2. Rename lib/irb/cmd/*.rb to lib/irb/command/*.rb
3. Rename test/irb/test_cmd.rb to test/irb/test_command.rb
4. Rename ExtendCommand to Command and alias it to Command
5. Rename Nop to Base and alias it to Base

[st0012]

I think we can maybe postpone the raise of deprecation warning. Otherwise, gems need to start requiring IRB release with this PR + start referencing to the new constants to avoid the warning. But at the same time, it won't bring immediate benefit to them, nor us, other than having clearer names in their codebase.

Update: I decided not to raise deprecation warning on old constants.

[tompng]

It looks like some repository is requiring "irb/cmd/nop". example: ruby/tracer, irbtoos search result

How about adding a file irb/cmd/nop.rb that only require irb/command/base?

[st0012]

@tompng That's a good point 👍
I added the file back but it actually doesn't need to require anything. In both tracer and irbtools (and I assume other tools too), they are requiring irb as well, which currently already loads the base command.