Use JSON mode when -f json is specified in a magic command

[JasonWeill]

Problem

Some generative language models do not generate valid JSON when asked to do so; this JSON fails when a user tries to parse it.

Proposed Solution

For models that support it (see below), use JSON mode when a user specifies -f json for a magic command.

Additional context

As of November 6, 2023, OpenAI's gpt-4-vision-preview and gpt-3.5-turbo models support JSON mode. Clients must include the string "JSON" in their system message, and clients using these models must set response_format to { type: "json_object" }, to enable this mode.

See #435 for another issue related to generative models that fail to output valid JSON.

See #437 for using JSON mode with the /generate command.