Add configuration to provide custom keybindings

[peytondmurray]

Summary

This PR allows the user to configure custom Vim-mode keybindings using the advanced settings editor, closing #43.

@mlucool I tested binding m in normal mode to :noh<CR>, and that seems to work. CodeMirror cannot fully emulate Vim, and many keys will collide with browser or JupyterLab keybindings, so for example if you try to bind a command to <Ctrl-/> it will not work. In any case marks are not supported in CodeMirror so m in normal mode should work just fine.

Changes

• Added the ability to create Vim keybindings in the advanced settings editor
• Set additionalProperties to false in plugin.json - it isn't being used here
• Refactored a bit of the logic inside updateSettings which deals with registering/unregistering commands, updating the tracker, and calling modifyCell to make it a bit simpler. This resolves a bug where the first call to updateSettings would succeed, but any subsequent call would not because it would try to re-register commands that were already registered.
• Updated the build matrix to remove node 12.x, and added node 18.x.
• My editor automatically runs JSON through jq (to automatically remove trailing , without me having to deal with it) which resulted in some reformatting of plugin.json. If this is a problem I can undo the reformatted lines.

[github-actions[bot]]

:point_left: Launch a binder notebook on branch peytondmurray/jupyterlab-vim/custom-keybindings

[krassowski]

error playwright@1.29.2: The engine "node" is incompatible with this module. Expected version ">=14". Got "12.22.12"

I would go ahead and drop node 12, it is EOL. Let's add node 16 or 18 to the matrix instead.

[krassowski]

Looking at Node.js 18 build failure, maybe we should try with 16 for now?

[peytondmurray]

Looks like node 16 works, thanks for the commit!