Enable sublime keymap from CodeMirror

[thayerve]

@paulproteus and I found that the commenting function called by Cmd-/ is one of several keyboard shortcuts included with the 'sublime' keymap of CodeMirror, so this commit just enables that whole shebang documented here: https://codemirror.net/demo/sublime.html

Also imports the Comment addon from CodeMirror because it's called in the keymap to toggle comment.

Caveat: This may not include Windows-friendly commands, as the cmd button is specified in the keymap.

Close #2276

[outoftime]

@thayerve thanks for this! I guess I’m not totally convinced that we would want to enable the entire Sublime keymap, can you elaborate on your thinking here? In particular:

• I would not expect many students who use Popcode to be familiar with Sublime and its keybindings; generally use of an editor like Sublime would come later in the student’s development as a coder
• A good number of the bindings in the sublime keymap seem to relate to functionality that we don’t have enabled for Popcode, e.g. code folding, bookmarks, marks, etc.

So my overall concern is that we’re pulling in a lot of keybindings that won’t work at all, and a lot more that will work but that probably no student will ever use. The main practical downside of this approach is that the intent isn’t clear (looking at the code, you’d have no way of knowing that what we are actually trying to accomplish is the addition of a multiline comment keyboard shortcut). If there’s a convincing case that there are a substantial number of other shortcuts that students in the bundle that students will find practically useful, I’m definitely open to it; I just don’t see much on a scan of the key binding list.

Let me know your thoughts!

[outoftime]

Superseded by #2386