Modernise build process and tooling

[AndrewIOM]

Motiviation

This PR includes a modernisation of the workflow for building and testing R.NET, including cross-platform CI and all projects on the latest SDK format. The library still targets netstandard2.0, so there should be no change to .NET framework compatibility. It should hpoefully make R.NET easier to develop and contribute bug fixes going forward.

As a maintainer on the RProvider project, I would like for R.NET to become a more active project again. We have various outstanding bugs and features that are dependent on changes in R.NET. Following this PR, I can send in further PRs for things we need for RProvider, or if current maintainers are low on time commitment I can potentially offer some time to help maintain the repo. This could offer some direction for re: #158. Thanks!

Changes

• FAKE build migrated to v6 (.NET 6 compatible version).
• Use FAKE to specify nuget package properties rather than nuspecs or proj files.
• All project files moved to .NET SDK format (if not already).
• Use paket for dependency management (removed all other package references).
• Set up Github Actions for CI - builds and tests (note: commented out in the Actions file is a part to automatically release nuget packages when a version is tagged on github. This is what we use in RProvider).
• Reorganised projects into src and test folders and removed redundant files related to .NET framework / mono building and other old build processes (e.g. nuget.exe process).
• Update readme to reflect the latest changes to the build and test process.
• Add status icons to readme for github actions and nuget.