Replace all/some uses of `GenericParent` and `GenericNode` with defined types

[agoose77]

The GenericNode and GenericParent types in MyST exist as an escape hatch for interacting with unified helpers like unist-util-select. However, over time they've spread throughout our codebase as types for intermediate state and function input/output types. Through using these generic types, we are not strict about whether something is "valid AST" or a superset of our AST, e.g. with user-added nodes.

I'm not 100% clear yet on what the ideal approach is. Sometimes we'll want to write generic tooling that allows user-defined node types. Other times, we are expecting a particular type.

The @types/mdast schema is closed (no support for unknown types) but it's possible to extend at compile time. Therefore, one approach is to write type signatures like

// Closed set of node
import type { Nodes } from 'myst-spec';

interface NodeLike {
  type: string;
}
interface ValueLike {
  type: string;
  value: string;
}
interface ParentLike {
  type: string;
  value: string;
}
type UnknownNode = NodeLike | ValueLike | ParentLike;

function processNode(node: Nodes | UnknownNode) {

}

which then allows us to write exhaustive switches on the node type.

Definition of Done

• We understand how we are replacing / refining GenericNode types
• We have performed a pass of the code-base to reduce type looseness from over-use of these types