feat: support unknown `EmbedField` properties by default

[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 unknown properties by default to EmbedField. It does this by adding a general [key: string]: unknown | null property to CommonEmbedData.

If a Data generic type is given, it overrides this new functionality.

This PR also adds a known common description property to CommonEmbedData. See: https://github.com/prismicio/prismic-client/issues/216

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

Merging #23 (dd7c80b) into master (4263c90) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           15        15           
=========================================
  Hits            15        15           

Impacted Files	Coverage Δ
src/fields.ts	100.00% <ø> (ø)

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