sqf: lint s31 - isNil/getVar

[PabstMirror]

ref acemod/ACE3#11083
Not sure about this, it's a little faster
but can't handle _z = nil; _z isNil "x" so it's not always safe to replace

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.6%. Comparing base (483f253) to head (d94719b).
⚠️ Report is 131 commits behind head on main.

Files with missing lines	Patch %	Lines
...ibs/sqf/src/analyze/lints/s31_isnil_getvariable.rs	25.2%	71 Missing ⚠️
...qf/src/analyze/lints/s26_short_circuit_bool_var.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
...qf/src/analyze/lints/s26_short_circuit_bool_var.rs	72.2% <0.0%> (ø)
...ibs/sqf/src/analyze/lints/s31_isnil_getvariable.rs	25.2% <25.2%> (ø)

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