Harden PAN & GSTIN validations

[miteshashar]

After review and merge of boxoffice.utils.misc.PAN_RE into main, it should be used for validation of PAN numbers.

GSTIN validations should incorporate the PAN validation as well.

[jace]

The only thorough way to validate both is via the online APIs; the regex and checksum approaches are just first pass. This is because PAN numbers have been issued with invalid checksums and those continue to remain valid.