message: Add restriction on `pos_end`
diff: @@ -127,10 +127,11 @@ Source Positions
  Any syntax object may optionally have position fields to reflect a source position:
  { ..., "pos": {"row": <int>, "col": <int>},
  - "end_pos": {"row": <int>, "col": <int>}?,
  + "pos_end": {"row": <int>, "col": <int>}?,
  "src": "<string>"? }
  The `pos` and `pos_end` objects have two keys: `row` (the line number) and `col` (the column number within the line). The `src` object can optionally provide the absolute path to a file which is referenced to by the source position.
  +If `pos_end` is provided, it must be equal to or greater than `pos`.
  Front-end compilers that generate Bril code may add this information to help with debugging.
  The [text format parser](../tools/text.md), for example, can optionally add source positions.
  However, tools can't require positions to exist, to consistently exist or not on all syntax objects in a program, or to follow any particular rules.
  