Add "leave-current-mode" command to move insert->normal->command.

[asford]

As we know, jupyterlab is already a "modal" editor and (by default) has a "edit4" and "command" mode. One switches between modes via Esc, a la vim.

It's rather nice to preserve this behavior when adding the vim-modes so that Esc conceptually moves "up" a mode, from vim-insert/lvisual to vim-normal and from vim-normal to jupyterlab-command.

Add a "Leave Current Mode" command, which can be rebound to Esc rather than "Leave Insert Mode".

This PR does not change the default Esc binding, which is separately proposed in #70.

Please finish the following when submitting a pull request:

• [x] Add a line to History.md briefly documenting the change.

If this is a release, additionally do the following:

• [x] Update the dependencies in package.json
• [x] Run jlpm install to update yarn.lock

Thanks!