feat: add custom type model types

[angeloashmore]

Types of changes

• [ ] 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 types for Custom Type models. The types reflect the JSON shape used to defined a Custom Type in Prismic.

The types would be used whenever interacting with the models directly. For example, the types could be used when using the Custom Types API. It could also be used in packages that parse Custom Type models.

Checklist:

• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly. (PR: #)
• [ ] All new and existing tests are passing.

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[codecov-commenter]

Codecov Report

Merging #2 (a38a098) into master (a2e41d6) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           31        31           
=========================================
  Hits            31        31           

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

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