Enable array of validations for AstNodes

[JohannesMeierSE]

At the moment, it is possible to register a single function for checking AstNode like this:

const checks: ValidationChecks<MyAstType> = {
  // ... checks for language-dependent types ...
  AstNode: checkAstNode
};

But it is not possible to register an array of functions for checking AstNode. This PR enables this feature:

const checks: ValidationChecks<MyAstType> = {
  // ... checks for language-dependent types ...
  AstNode: [ checkAstNodeOne, checkAstNodeTwo ]
};

This behaviour is already supported for language-dependent types, but not for AstNode yet.