ast: error recovery

[pgavlin]

Allow the parser to recover from errors and return its best possible AST. This is the first step towards allowing evaluation of environments that contain parse errors, which is vital for live analysis.

[pgavlin]

follow-up changes will update the evaluator to handle the new AST patterns, specifically:

• interpolations that contain errors
• fn::open calls that contain errors
• fn::join calls that contain errors
• fn::secret calls that contain errors