Add suggestions/snippets to the model

### 🤔 What's the problem you're trying to solve?

Snippets are not currently part of the messages schema. As we move to messages-based formatters, they won't be able to include snippets unless we add them to the schema. This is already the case with the xml, pretty and html formatters.

### ✨ What's your proposed solution?

Add snippets to the messages schema.

There's more than one good way we could do this, but my first draft would be:

- New `Snippet` message emitted once for each unique snippet containing
- `id`
- `text` the unmatched step text
- `code` the suggested code
- `language` the language of the suggested code (useful for e.g. syntax highlighting in formatters)
- New optional field `snippetId` on the existing `TestStepResult` message, to be included for undefined steps where there is a snippet