git rebase -i <base> can currently only pick commits. I'm planning on changing the pick button into some kind of mutually-exclusive slider (or set of radio buttons) so that you can either pick a commit or squash a commit.

Do you have any advice on which files I would need to edit to add squash functionality?
I was looking at src/js/git/commands.js and I suspect I'll need to change something in this section:

 rebase: {
    sc: /^gr($|\s)/,
    options: [
      '-i',
      '--solution-ordering',
      '--interactive-test',
      '--aboveAll',
      '-p',
      '--preserve-merges'
    ],
    regex: /^git +rebase($|\s)/,
    execute: function(engine, command) {
      var commandOptions = command.getOptionsMap();
      var generalArgs = command.getGeneralArgs();

      if (commandOptions['-i']) {
        var args = commandOptions['-i'].concat(generalArgs);
        command.twoArgsImpliedHead(args, ' -i');

        if (commandOptions['--interactive-test']) {
          engine.rebaseInteractiveTest(
            args[0],
            args[1], {
              interactiveTest: commandOptions['--interactive-test']
            }
          );
        } else {
          engine.rebaseInteractive(
            args[0],
            args[1], {
              aboveAll: !!commandOptions['--aboveAll'],
              initialCommitOrdering: commandOptions['--solution-ordering']
            }
          );
        }
        return;
      }

      command.twoArgsImpliedHead(generalArgs);
      engine.rebase(generalArgs[0], generalArgs[1], {
        preserveMerges: commandOptions['-p'] || commandOptions['--preserve-merges']
      });
    }
  },

I'll also need to edit the pop-up interactive rebase dialog so that instead of a pick button it has the mutually-exclusive slider/radio buttons.