ci: add lock file to repository

[mtrezza]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Related issue: #749

Approach

• Remove pubspec.lock from .gitignore so that it is checked in to this repository for development.
• Add pubspec.lock to .pubignore so that it is not published in the registry as part of the package.

TODOs before merging

n/a

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov[bot]]

Codecov Report

Merging #750 (c749701) into master (d80ca58) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #750   +/-   ##
=======================================
  Coverage   10.28%   10.28%           
=======================================
  Files          48       48           
  Lines        2810     2810           
=======================================
  Hits          289      289           
  Misses       2521     2521           

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[fischerscode]

The failing test is fixed as part of #734.