Add support for a completely different design system (e.g., the NHS)

### What feature do you want to see added?

Add support for a completely different design system to GOV.UK for the user to select and use for their prototype.

The NHS has a completely separate design system that also uses Nunjucks components, see https://service-manual.nhs.uk/design-system.

See other design systems at https://github.com/ctdesign/gov-design-systems-list.

### Why do you want this feature? Does it solve a problem?

Currently there is also the HMRC design system (in addition to GOV.UK), however that is the same as GOV.UK with the HMRC logo. It'd be beneficial to show how design systems can be swapped out for each other and still use the same base JSON.

### How should the feature be implemented?

Currently there is a single set of code for generating a prototype as per a given design system. This should be duplicated into separate code for GOV.UK and separate code for another design system.

The code to generate a prototype (for any design system) should not be coupled to code for a specific design system. It should be clear how additional design systems could be easily added in the future. Essentially, swapping design systems in or out should require minimal changes to the core application beyond adding the design system-specific implementation.

Consider how the prompts to the LLM may need to be adjusted to account for a different organisation and context.

Also consider splitting out HMRC into a separate design system (even though it is fundamentally the same) for consistency. Issues with duplication would need to be resolved.

Be careful about using organisational logos or fonts. It may be more appropriate to not incorporate them in a prototype, and caveat that this is a proof-of-concept only.

### Additional context (if applicable)

_No response_

Start by reviewing the current single design-system prototype generator and the prompts that define its organisation and context. Compare the NHS design system's Nunjucks components with the existing GOV.UK and HMRC approach. Done means a user can select another design system, the core generator is not coupled to one system, and adding future systems requires minimal core changes.

Written by the indexing model from the issue text.