feat: export PackageOwnership type

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Export PackageOwnership(Form) Type #24
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Applies the cleanup as suggested.

[codecov]

Codecov Report

Merging #25 (d4150c0) into main (3a35abc) will not change coverage.
The diff coverage is 100.00%.

❗ Current head d4150c0 differs from pull request most recent head 4d0cc84. Consider uploading reports for the commit 4d0cc84 to get more accurate results

@@            Coverage Diff            @@
##              main       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         8    -1     
  Lines           43        43           
  Branches        10        10           
=========================================
  Hits            43        43           

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

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