Consider if expression for parenthesized with items parsing

[dhruvmanila]

Summary

This PR fixes the bug in parenthesized with items parsing where the if expression would result into a syntax error.

The reason being that once we identify that the ambiguous left parenthesis belongs to the context expression, the parser converts the parsed with item into an equivalent expression. Then, the parser continuous to parse any postfix expressions. Now, attribute, subscript, and call are taken into account as they're grouped in parse_postfix_expression but if expression has it's own parsing function.

Use parse_if_expression once all postfix expressions have been parsed. Ideally, I think that if could be included in postfix expression parsing as they can be chained as well (x if True else y if True else z).

Test Plan

Add test cases and verified the snapshots.

[github-actions]

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