feat(js/ai): allow disabling native constrained generation and enabling custom instruction

[pavelgj]

    const { output } = await generate(registry, {
      prompt: 'generate json',
      output: {
        constrained: false,
        instructions: true,
        schema: z.object({
          foo: z.string(),
        }),
      },
    });

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)