refactor: Remove INavigable in favor of IFocusableNode.

[gonfunko]

The basics

• I validated my changes

The details

Proposed Changes

This PR gets rid of the INavigable interface, since it was reduced to just getClass(), and there was significant objection to that. Instead, we now act directly on IFocusableNode instances. As a side effect, this also makes navigation policies Just Work for Field subclasses, and opens up the possibility of removing the cursor's curNode in favor of asking the focus manager for the currently focused thing, although I'll address that in a followup change. Registering navigation policies has been moved out of the cursor and into the Navigator.

[BenHenning]

Thanks @gonfunko! Had some thoughts.

Approving to unblock, but I'm happy to take another pass. Didn't look at the flyout navigation policy super closely since it appeared to have a bunch of unfinished code int.

[BenHenning]

A nit but I don't think this actually returns a boolean, I think it returns current coerced to type T or undefined if coercion fails per the predicate.

I'm not 100% certain on this and I find the TypeScript documentation unclear, but I recall debugging guards like this before and being surprised that their return values weren't actually booleans.

[gonfunko]

I agree the docs are unclear on this, but I think boolean is more useful to both folks reading and implementing than the perhaps technically correct value; ultimately the method needs to return a boolean (if you're implementing it) and you can rely on it being a truthy value if you call it.

[BenHenning]

I used the language 'whether' in this case which I suppose is slightly more ambiguous (perhaps beneficially):

/**
 * Determines whether the provided object fulfills the contract of
 * IFocusableNode.
 *
 * @param object The object to test.
 * @returns Whether the provided object can be used as an IFocusableNode.
 */
export function isFocusableNode(object: any | null): object is IFocusableNode {
  return (
    object &&
    'getFocusableElement' in object &&
    'getFocusableTree' in object &&
    'onNodeFocus' in object &&
    'onNodeBlur' in object &&
    'canBeFocused' in object
  );
}

Not sure that's actually better. Note I don't have strong opinions here, so if you prefer the explicit 'True' that seems reasonable to me.

[rachel-fenichel]

Approved mod some deletion of logs.

This feels solid.