feat(semver): enforce specifiers like ^1 or ~1.2

Nobody has claimed this yet.

### Question

## Context

Currently, when using `semverGroups` with `"range": "^"`, syncpack always produces the full `^x.y.z` format (e.g. `^1.0.0`).
Node-semver also considers `^1` and `^1.2` as valid caret ranges — they're semantically equivalent to `^1.0.0` and `^1.2.0` respectively, but some teams prefer the shorter forms.

## Request

Would it be possible to support a range value like `"^.x"` or similar that formats versions as caret + major only?

For example:
| Current (`"range": "^"`) | Requested |
|---|---|
| `^1.0.0` | `^1` |
| `^2.0.0` | `^2` |

## Use case

For internal monorepo packages where the intent is "any compatible version within this major", the shorter `^1` form communicates that intent more directly, it says "I don't care about a specific minor or patch floor, just keep me on major 1". The full `^1.0.0` can imply a deliberate choice of `1.0.0` as the minimum, when in practice the consumer just wants major-level compatibility.

I'd love your take on whether this is even a good idea.

### Code of Conduct

- [x] I agree to follow the [Code of Conduct](https://github.com/JamieMason/syncpack/blob/main/CODE_OF_CONDUCT.md)

Start by locating the semverGroups range handling and the tests covering existing "^" formatting. Clarify the configuration syntax and expected behavior for major-only and major-minor forms, then add coverage showing that the selected format is emitted consistently.

Written by the indexing model from the issue text.