Currently, invalid shapes cause an error when rendering, but not when
parsing. This confuses the Mermaid Live Editor, e.g. by not showing the
error message.
Instead, I've added an `isValidShape()` that validates if the shape is
valid at parse time. This only checks shapes using the new extended
shapes syntax. Currenlty, using `A(-this is an ellipse node-)` is broken
(see #5976) and also causes an invalid shape error at render time, but
I've ignored it in this PR, so it will continue pass at parse-time
(we have unit tests checking ellipse parsing).
See: https://github.com/mermaid-js/mermaid/issues/5976
