Modernize build 2021-08

[quintusm]

This pull request starts the process of modernising the build and documentation for Excelprovider.

• Upgrades FAKE to version 5
• Use fake as dotnet tool
• Use paket as dotnet tool
• Only provide ExcelProvider as NetStandard2.0 dlls. This provides maximum compatibility and lower maintenance costs. This change is the reason I suggest bumping the version number to 2.0-rc1.

Not all the build targets are updated yet, so not all old functionality is enabled yet in this pull request. I have additional changes lined up to re-enable most of the remaining targets.

I will publish the version2.0-rc1 nuget package when this request is merged into master

[sergey-tihon]

I think that would be nice to replace Travis and AppVeyor by GitHub Actions in space of build modernization - here is sample workflow if you need it

[quintusm]

Thank you @sergey-tihon. That is certainly where I am headed. Appreciate the sample fragment!