feat: support image captions

[angeloashmore]

Types of changes

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

Description

This PR adds support for image captions. It does this by:

• Add a boolean allowCaption config property to the Image widget.
• Add a caption property to ImageContent that holds string | null.
• Ensures the caption image property is not treated as a thumbnail in asImageWidthSrcSet.

The changes in this package mirror the changes in @prismicio/types-internal. See https://github.com/prismicio/prismic-types-internal/pull/69 for details.

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

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.97%. Comparing base (bd55071) to head (9cb5ae7).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #334      +/-   ##
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- Coverage   99.97%   99.97%   -0.01%     
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  Files          61       61              
  Lines        6885     6881       -4     
  Branches      381      381              
==========================================
- Hits         6883     6879       -4     
  Misses          2        2              

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