[ruby/irb] Introduce cd command

(https://github.com/ruby/irb/pull/971)

It's essentially a combination of pushws and popws commands that are
easier to use.

Help message:

```
Usage: cd ([target]|..)

IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
The `cd` command is an attempt to simplify the operation and will be subject to change.

When given:
- an object, cd will use that object as the new context by pushing it onto the workspace stack.
- "..", cd will leave the current context by popping the top workspace off the stack.
- no arguments, cd will move to the top workspace on the stack by popping off all workspaces.

Examples:

  cd Foo
  cd Foo.new
  cd @ivar
  cd ..
  cd
```

https://github.com/ruby/irb/commit/4a0e0e89b7

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