Add substantive content for algorithms and definitions (#199)

Make a series of refinements to the Prompt API spec, including:
- Update intro
- Define algorithms aligned with, and leveraging, writing-assistance infra
- Define language model object, internal slots, attribute getter steps and events
- Define prompt(), promptStreaming(), and append() using shared prefill and generate steps
- Validate and normalize message sequences from inputs, handle empty sequences
- Compute context usage around initial prompts and constraint inputs
- Enforce system-role message limitations

Author: michaelwasserman