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