Make completion options overridable

[squld]

Override Options

Customize Markdown Copilot's behavior with override options. This allows you to control settings like response length or the AI model directly within your document.

To use override options, simply include a JSON code block labeled json copilot-options with your desired settings, then select this block along with your text and choose 💡 Markdown Copilot: Continue from the code action proposals.

Example: Let Markdown Copilot introduce itself with customized response length and model:

Introduce yourself.

```json copilot-options
{"max_tokens":50,"model":"gpt-3.5-turbo"}

For more configuration options, please refer to the [OpenAI API: Create chat completion](https://platform.openai.com/docs/api-reference/chat/create).