By retargeting PSReadLine to `netstandard2.0`, we can simplify the build a lot and produce the same assembly no matter building on Windows or non-Windows.
- Update PSReadLine build to target `netstandard2.0`, including the CI and release pipeline YAML files.
- Update the test to target `net472` and `net6.0`, so that we can run tests with both .NET Framework and .NET
- Rename `Microsoft.PowerShell.PSReadLine2.dll` to `Microsoft.PowerShell.PSReadLine.dll`
- Update and clean up the `README.md` with the up-to-date information
This PR refactors the building scripts and fix the AppVeyor CI. Major changes are:
1. Add `build.ps1` to provide simple entry points that can be used in CI.
2. Add `tools/helper.psm1` to group utility functions. Move the scripts for `RunTests` to `helper.psm1`. The plan is to move big chunk of build action scripts to the `helper.psm1` to make `PSReadLine.build.ps1` less cumbersome but more clear about what build actions are available.
3. Fix the AppVeyor CI.
Justin Chung