This PR adds a lock file and enforces it to be used when restoring nuget packages in a CI pipeline.
This gives more visibility about what packages change when a package reference is changed.

https://devblogs.microsoft.com/dotnet/enable-repeatable-package-restores-using-a-lock-file/

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.
• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

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• I have read the Contributor License Grant and accept the conditions
• I'm interested in a free license for Fluent Assertions and will share my email address through sales@xceed.com