tab and shift+tab should move between indent levels about parinfer.js

[shaunlebron]

This is larger in scope than I thought. Providing any auto-indent feature may hurt the overall initial experience, since it may create an expectation of more auto-indent features, for example:

• auto-indentation of atoms seems straightforward
  • join the collection of the sibling above when indenting, or become a sibling of its parent collection
• auto-indentation of collections will behave similarly, but we must decide whether to indent its children with it or not.
• we must also indent differently for lists depending on whether the first argument is on the first line.

I feel it's a slippery slope adding auto-tabs because it moves us from character-based manipulation to an implied structural one. And structural manipulation implies more work of course, not just on development, but on the user's ability to understand the rules of this system.

Closing this for now while we keep this initial concept simple.