Is there any type guards?

Is your feature request related to a problem? Please describe. I imported plantuml-parser in my Next.js app, and I need some of type guards. Now I just written a piece of type guards(only for what I need), and they are in my app project.

eg. When I render UML in browser,

classify between interface and class to show some icon
classify between methods and member variable to separate div tag

Describe the solution you'd like

As far as I have been able to find in the repository, it does not appear that type guards are provided. If I am mistaken and type guards already exists in the project, please let me know.
May I create a pull request to add the type guard I coded? It might help other users. (although it may not be necessary) Currently I have only created type guards for a few classes I need, but I consider that I can create type guards for all types defined in types.d.ts.

Here is an example of what I have created;

const isMethod = (maybeMethod: Object): maybeMethod is Method => {
  return (
    hasPrimitive(maybeMethod, 'name', 'string') &&
    hasPrimitive(maybeMethod, 'isStatic', 'boolean') &&
    hasStringLiteral(maybeMethod, 'accessor', isAccessor) &&
    hasPrimitive(maybeMethod, 'returnType', 'string') &&
    hasPrimitive(maybeMethod, '_arguments', 'string')
  );
};

const isMemberVariable = (maybeMemberVariable: Object): maybeMemberVariable is MemberVariable => {
  return (
    hasPrimitive(maybeMemberVariable, 'name', 'string') &&
    hasPrimitive(maybeMemberVariable, 'isStatic', 'boolean') &&
    hasStringLiteral(maybeMemberVariable, 'accessor', isAccessor) &&
    hasPrimitive(maybeMemberVariable, 'type', 'string')
  );
};

const isMember = (maybeMember: Object): maybeMember is Member => {
  return isMethod(maybeMember) || isMemberVariable(maybeMember);
};

// how to use
const renderClass = (c: Class) => {
  const variables = c.members.filter(member => isMemberVariable(member));
  const methods = c.members.filter(member => isMethod(member));

  return <div className="container">
    <h1>{c.title}</h1>
    {variables.length > 0 &&<div className="properties">
      <h2>properties</h2>
      <ul>{variables.map(v => <li>{v.name}</li>)}</ul>
    </div>}
    {methods.length > 0 &&<div className="methods">
      <h2>methods</h2>
      <ul>{methods.map(m => <li>{m.name}</li>)}</ul>
    </div>}
  </div>
}

Enteee / plantuml-parser

Is there any type guards? #92