fix(parser): parse IS/ISNULL/NOTNULL with lower precedence than comparison

[ansh-rohilla]

In standard SQL and PostgreSQL operator precedence, comparison operators (=, !=, <>, <, >, <=, >=) have higher precedence than IS, IS NOT, ISNULL, and NOTNULL.

Previously in SQLGlot, IS was registered in RANGE_PARSERS under _parse_range(), which sits underneath _parse_comparison() and _parse_equality(). Consequently, in an expression like t.a = t.b IS NULL, t.b IS NULL was evaluated first as the right-hand operand of =.

This PR introduces _parse_is_predicate() in the parser between conjunctions (AND) and equality/comparison (_parse_equality()), and removes IS from RANGE_PARSERS.

Closes #8661

[georgesittas]

Thanks, but the core team will take a look at this one.