fix: restore group field item type exports

[angeloashmore]

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR restores exports for a group field's item interface.

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a020818) 97.11% compared to head (843fdbd) 97.11%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #60 +/- ## ======================================= Coverage 97.11% 97.11% ======================================= Files 20 20 Lines 1490 1491 +1 Branches 161 161 ======================================= + Hits 1447 1448 +1 Misses 42 42 Partials 1 1 ``` | [Files Changed](https://app.codecov.io/gh/prismicio/prismic-ts-codegen/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio) | Coverage Δ | | |---|---|---| | [src/lib/buildFieldProperties.ts](https://app.codecov.io/gh/prismicio/prismic-ts-codegen/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=prismicio#diff-c3JjL2xpYi9idWlsZEZpZWxkUHJvcGVydGllcy50cw==) | `97.11% <100.00%> (+<0.01%)` | :arrow_up: |

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