add an ace mode for `.sc` files

[pq]

currently non-lua files default to a generic text editor but a custom mode for supercollider would be great. as of yet, noone has written one so this would be on us.

things the mode could provide include:

• syntax highlighting
• basic parsing and error reporting
• indent smarts
• snippets and code completions

[mhetrick]

I opened an additional norns-wide issue for better SC tooling, so it might be good to cross-reference it here: monome/norns#462

In addition to providing a method for editing ".sc" files in maiden, it would be useful to have the ability to restart the audio engine from maiden without restarting the entire device from the hardware.

[ngwese]

sc files are now exposed for simple editing. full editing support is likely best accomplished in the sc ide