Analyzer: Report inefficient use of sets.

Similar to https://github.com/dotnet/runtime/issues/33799, use of sets can be optimized. Redundant uses of Contains can be removed around use of .Add and .Remove methods.

```csharp
if (!set.Contains("Foo")) set.Add("Foo");
if (set.Contains("Foo")) set.Remove("Foo");
```

No need for the calls to Contains in the above.

Suggested category: Performance
Suggested severity: Info