Fix __defineGeneric not a function

[Brianzchen]

Closes #49 by rewriting the way we identify and then add flow types as defined to the eslint scope

[pieterv]

@Brianzchen context.sourceCode can be undefined in older versions of eslint so this code causes the following error: TypeError: Cannot read properties of undefined (reading 'getScope').

I think you want something like:

      if (context.sourceCode != null && context.sourceCode.getScope != nul) {
        globalScope = context.sourceCode.getScope(node);
      } else {
        globalScope = context.getScope();
      }