feat(2d): add querying helpers

[aarthificial]

Introduces multiple helpers for querying the node hierarchy and dealing with type casting.

Node.findAll()

Finds all descendants of the node that match the given predicate:

const texts = node.findAll<Txt>(child => child instanceof Txt);

Node.findFirst()

Finds the first descendant that matches.
Uses depth-first traversal, like querySelector in DOM.

Node.findLast()

Finds the last descendant that matches.
Order is the same as in findFirst but in reverse.

Node.findAncestor()

Finds the first ancestor that matches.

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is()

A helper function for creating instanceof predicates.
It's less verbose than manually writing instanceof and, thanks to type guards, TypeScript can infer the type of the resulting array automatically:

const texts = node.find(is(Txt));

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Node.[child/children/parent]As()

A set of helpers for easier casting. They remove the need for a (...) wrapper that stems from using as.
Solely for convenience, they don't actually use instanceof to type-check anything:

// old way:
(node.children()[2] as Txt).fill('white');
// new way
node.childAs<Txt>(2).fill('white');

// old way:
yield* all(...node.children().map(child => (child as Txt).fill('white', 2)));
// new way
yield* all(...node.childrenAs<Txt>().map(child => child.fill('white', 2)));

// old way:
(node.parent() as Txt).fill('white');
// new way
(node.parentAs<Txt>().fill('white');