Fix tests and add GitHub Action workflows

[Shane32]

This PR fixes the tests so the run correctly on Linux and Windows. It also includes GitHub Action workflows to run tests during pull requests to test the code and documentation on the following platforms:

• .NET Framework 4.6.2
• .NET Framework 4.8
• .NET Core 2.1
• .NET Core 3.1
• .NET Core 5.0
• .NET Core 6.0

All but one of the tests works properly. The one test that fails is the CrossSheetInsertRowsUpdatesReferencesCorrectly test within WorkSheetTests.cs. It fails only when running on Linux; it works correctly across all platforms on Windows. This is probably a bug that needs fixing. Note that many of the existing tests required a xlsx file to test against, which are not included in this repository, and as such were previously disabled. I did not touch these tests; they are still disabled.

There is also a GitHub Action included to build the EPPlusFree package (nupkg file) and all documentation upon pushing to master (either directly, or indirectly via a merged pull request). The nupkg and docs are uploaded as artifacts to the workflow. In addition, NuGet package is uploaded to the GitHub Package Repository as a beta version.

I can also write a GitHub Action script to publish the nupkg file to NuGet.org upon issuing a release, using the version number associated with the release. The nupkg and docs can be automatically attached to the release as release artifacts. As such, releasing a new version to NuGet.org requires only a couple clicks from the GitHub website by a collaborator. Please let me know if you are interested; otherwise I will just publish my own fork to NuGet.

Note that this PR includes and requires the two prior PRs:

• Fix calculations on .NET Framework #2
• Fix documentation project #3