the type definition to check.
Optionalkey: stringthe key to get the type for, which can be a simple key or a nested key using dot notation.
Optionalarray_path: booleana flag indicating if the current path is within an array.
the type associated with the key, or undefined if the key is not found.
Get the defined type for a given key in a type definition, supporting nested keys using dot notation (e.g., "person.name.first"). TODO: Support arrays in the path (e.g., "person.children.name").