Use type guards for `language.predicates`

[nrbnlulu]

currently 3.2.3 implemented like this

def is_definition_node(node: Node) -> bool:
    """Check whether the given node represents a definition."""
    return isinstance(node, DefinitionNode)

def is_executable_definition_node(node: Node) -> bool:
    """Check whether the given node represents an executable definition."""
    return isinstance(node, ExecutableDefinitionNode)

[nrbnlulu]

whops seems like this is solved in #183