Improved error recovery #36
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This PR vastly improves the error-recovery capabilities of the parser.
Previously, a parse error occurring in any part of a top-level declaration would cause the parser to bail out of that entire declaration and pick up again at the next one. That limited form of error recovery was better than nothing, but it was liable to cause some UX issues as, e.g., "Go to definition" requests would fail for a function declaration if any single statement in the function body was ill-formed.
This PR applies the same technique that was previously used for top-level declarations to every heterogeneous sequence of nodes in the tree. This includes things like block statements, argument and parameter lists, field declarations, etc. Now, for example, if one argument of a function-call expression emits a parse error, only that argument will be missing from the tree.