Brackets check doesn't account for arbitrary nested parentheses

[chisholm]

There is code to add a simpler error message in one particular case: when the user has forgotten square brackets around the pattern. But it is too specific: it requires that patterns start with either [ or ([:

cti-pattern-validator/stix2patterns/v21/validator.py

Line 98 in 2dafa44

if not (start[0] == '[' or start == '(['):

Patterns can have arbitrarily nested structure, so the pattern can start with an arbitrary number of open parentheses. The check needs to be generalized.