Validate tasklet subscript access dimensionality

[tbennun]

This PR adds a validation check that ensures connector access in tasklets conforms to the SDFG IR rules. This gives a more verbose and clearer error than before, where it would either manifest in the code generator or remain undetected.

cc @acalotoiu @pratyai @mcopik

[mcopik]

@tbennun Thank you, LGTM!