(https://github.com/ruby/irb/pull/888)

* Remove dead irb_level method

* Restructure workspace management

Currently, workspace is an attribute of IRB::Context in most use cases.
But when some workspace commands are used, like `pushws` or `popws`, a
workspace will be created and used along side with the original workspace
attribute.

This complexity is not necessary and will prevent us from expanding
multi-workspace support in the future.

So this commit introduces a @workspace_stack ivar to IRB::Context so IRB
can have a more natural way to manage workspaces.

* Fix pushws without args

* Always display workspace stack after related commands are used

https://github.com/ruby/irb/commit/61560b99b3

(https://github.com/ruby/irb/pull/885)

https://github.com/ruby/irb/commit/f6d489658e

(https://github.com/ruby/irb/pull/881)

https://github.com/ruby/irb/commit/83d90550c2

(https://github.com/ruby/irb/pull/882)

* Unroll extension method generation

Given we only have 2 remaining extension setter methods, both of which
only take 1 argument and don't have any alias, the current method generation
logic is overly complicated.

This commit simplifies the method generation logic by simply defining
the methods directly in the `IRB::Context` class.

* Fix use_loader extension

https://github.com/ruby/irb/commit/67eba5401b

(https://github.com/ruby/irb/pull/873)

* Replace ExtendCommand with Command and standardize command related 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

* Alias ExtendCommand to Command and deprecate it

* Rename Command::Nop to Command::Base

* Not deprecate old constants just yet

* Add lib/irb/cmd/nop.rb back

https://github.com/ruby/irb/commit/462c1284af

https://github.com/ruby/irb/commit/cef125850d

https://github.com/ruby/irb/commit/07fae94862

https://github.com/ruby/irb/commit/0b2773d91d