changes syntax for EXISTS

  was:

    if x exists
      span do something

  now:

    if exists x
      span do something
fixes bug in EXISTS when syntax changed
fixes bug in nested forms for indexing/slicing
updates PathNode to be single function
adds string interpolation

  - adds MananaStringNode()
  - renames editor to IDE
  - regenerates parser
  - updates PathNode() for string interpolation purposes (so that IdNode() & PathNode() can be called from interpreter)