Support analyzer 14 (widen constraint to <15.0.0)

## Summary

Please widen `source_helper`'s `analyzer` constraint to allow analyzer 14.

Current (`source_helper` 1.3.12):

```yaml
analyzer: '>=9.0.0 <14.0.0'
```

Latest `analyzer` on pub.dev is `14.1.0`. This blocks dependents such as `json_serializable` (which depends on `source_helper: ^1.3.10`) from supporting analyzer 14 even if they widen their own constraint.

## Requested change

- Bump `analyzer` to something like `'>=9.0.0 <15.0.0'` (following prior bumps, e.g. [#69](https://github.com/google/source_helper.dart/pull/69) for v1.3.12).
- Publish a new `source_helper` release.

## Context

`source_gen` 4.2.4 already allows `analyzer: '>=8.1.1 <15.0.0'`. A matching `source_helper` bump would unblock the analyzer 14 upgrade path for `json_serializable` and downstream packages (e.g. `hive_ce_generator`).

Related: https://github.com/google/json_serializable.dart/issues/1580

Thanks!

Start by locating the analyzer dependency constraint shown for source_helper and review the prior constraint bump in pull request #69. Done means the constraint allows analyzer 14, dependents can resolve the package, and a new source_helper release is published.

Written by the indexing model from the issue text.