Handling and tests for expression trees

[AArnott]

These tests repro the cases called out in #3, plus a couple more.

Fixes #3

[codecov-commenter]

Codecov Report

Merging #4 (9dd2636) into main (09d8a20) will increase coverage by 0.89%.
The diff coverage is 98.92%.

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   94.08%   94.97%   +0.89%     
==========================================
  Files           7        8       +1     
  Lines         169      219      +50     
==========================================
+ Hits          159      208      +49     
- Misses         10       11       +1     

Impacted Files	Coverage Δ
src/CSharpIsNullAnalyzer/IOperationExtensions.cs	92.30% <92.30%> (ø)
...CSharpIsNullAnalyzer.CodeFixes/CSIsNull002Fixer.cs	100.00% <100.00%> (ø)
src/CSharpIsNullAnalyzer/CSIsNull001.cs	100.00% <100.00%> (ø)
src/CSharpIsNullAnalyzer/CSIsNull002.cs	100.00% <100.00%> (ø)

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