Add support to specify the type returned by query functions

[Gudine]

I've made the query functions (query, useQuery, useCurrentFile, etc.) accept a generic to specify its return type, much like the built-in querySelector type.

To make that more useful, I also added a way to specify the types of fields in classes with fields, useful in cases where you know what fields pages or sections returned by the query has. For example:

type PersonPage = MarkdownPage.Typed<{ name: string, age: number }>;

return function View() {
  const person = dc.useCurrentFile<PersonPage>();

  // Automatically typed as string
  const name = person.value("name");

  // Automatically typed as number
  const age = person.value("age");

  // Error: Argument of type '"height"' is not assignable to parameter of type '"name" | "age"'
  console.log(person.value("height"));

  return `${name}, ${age} years old`;
}

[blacksmithgu]

Thank you!