Type and documentation improvements to JSON-LD API

[multimeric]

Summary of changes

All these changes should have no impact at runtime, they're purely to improve UX through static analysis.

• Convert Term to a typed NamedTuple from a namedtuple. This makes static analysis easier in that your IDE and type checker can understand which fields are present on the type, the type of each field, and give each field its own documentation
• Document the meaning of each field in Term for usability
• Move the type definition of Context fields into the class body. This is also for static analysis and allows the user to get tab completion etc when using these fields.

My long term goal is to make this context API usage as a public API, so that I can generate and interact with contexts programmatically instead of just by defining these in JSON.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.