Suppress SC2319/SC2320 when $? is assigned to a variable

[ayushi-work]

Summary

Fixes #3486

SC2319 warns "This $? refers to a condition, not a command. Assign to a variable to avoid it being overwritten." However, it fired even when $? was already being assigned to a variable via || or &&, like:

[[ -e "./lockfile" ]] || retval=$?

The user is already following the advice (assigning to a variable), so the warning is a false positive.

This change skips both SC2319 and SC2320 when $? appears inside a T_Assignment node.

Test plan

• cabal test passes (all existing + 3 new prop_ tests)
• The issue snippet no longer triggers SC2319
• Non-assignment cases like exit $? after a condition still trigger SC2319

[ayushi-work]

Hi @koalaman, just bumping this up!