Stop generating new singleton methods for abstract classes

[paracycle]

Motivation

Fixes #1507

Implementation

When generating methods for a constant, we check to see if:

1. the constant is a singleton class (does it have an attached class?)
2. the method is named :new
3. the constant is marked as :abstract or :interface
4. the constant is a Class

If all these conditions are satisfied, then we skip the definition of the new method, since generating it messes with Sorbet's internal handling of how new method calls behave.

Tests

Added a new test and updated existing tests to reflect the behaviour change.