Add optional AST node type

[msujew]

The proposed OptionalAstNode<T> is a generic type that sets all properties, that could be potentially missing due to parser errors, to be optional. However, this excludes specificially:

• Properties inherited from AstNode (mainly the $type property which is always set).
• All boolean properties. Those will be set to false by the parser, even if the property wasn't parsed.
• All array property. Those will be set to [] by the parser.

This change adjusts the arithmetics language validator to use that new type to ensure that all properties are properly checked for truthyness before they are accessed. Inspired by the discussion in #1758.

[sorawee]

Thanks!

Would it make sense to make this recursive too? I believe a child node that we want to recur downward is either AstNode[] or AstNode | undefined or AstNode. Do I miss anything else?

Here's my attempt to take a stab at it. (This is the first time in my life that I do type-level programming actually! Apologies if anything is off)

export type OptionalAstNode<T extends AstNode> = {
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  [P in keyof T]: 
    P extends keyof AstNode ? 
      T[P] : 
      T[P] extends AstNode[] ? 
        OptionalAstNode<T[P][number]>[] : 
        T[P] extends any[] | boolean ? 
          T[P] : 
          T[P] extends AstNode | undefined ? 
            OptionalAstNode<Extract<T[P], AstNode>> | undefined :  
            (T[P] extends AstNode ? OptionalAstNode<T[P]> : T[P]) | undefined;
};