Show async as one of the features on llm logs --options

[simonw]

Following:

• Add a note to llm models showing which models support schemas #796

It should look like this:

OpenAI Chat: gpt-4.5-preview (aliases: gpt-4.5)
  Options:
    temperature: float
    max_tokens: int
    top_p: float
    frequency_penalty: float
    presence_penalty: float
    stop: str
    logit_bias: dict, str
    seed: int
    json_object: boolean
  Attachment types:
    image/gif, image/jpeg, image/png, image/webp
  Features:
  - streaming
  - schemas
  - async

[simonw]

Example at bottom of: https://github.com/simonw/llm/blob/b829cd92e098cfe8d894f4b1f35f4f43dc464edd/docs/usage.md#listing-available-models