Implement parser that supports unsupported features

When writing CakeML code/porting code from other ml implementations one would accidentally use features that CakeML currently doesn't support. This issue is not proposing to implement these features but make the parser successfully parse said features and output error messages along the lines that the feature isn't supported.

The issue names no files, tests, or parser entry point. First identify the parser and agree on which unsupported features are in scope; done means those constructs parse and produce explicit unsupported-feature errors without implementing them.

Written by the indexing model from the issue text.