chore: export missing user models in library file

[robertodoering]

1. Description

This PR adds exports for a couple of public-facing user models in the lib/twitter_api_v2.dart library file.

1.1. Checklist

• [x] The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs: etc).
• [x] I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples.

1.2. Breaking Change

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.

[codecov-commenter]

Codecov Report

Merging #585 (05fd0fe) into main (07a3658) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #585   +/-   ##
=======================================
  Coverage   87.54%   87.54%           
=======================================
  Files         154      154           
  Lines        1887     1887           
=======================================
  Hits         1652     1652           
  Misses        235      235           

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[myConsciousness]

Hi @robertodoering ,

Thanks for your contribution, it's very helpful! :)

[myConsciousness]

bors merge

[bors[bot]]

Build succeeded:

• analyzer
• test