Add ImageSharp renderers for .net 6.0, 5.0 and standart 2.0

[AntonKorn]

Summary

This PR adds the ImageSharp cross platform library to the QRCoder. It helps to still use Base64QRCode and QRCode in linux or any other environment. New rederers are only included in .net 6.0, 5.0 and standart 2.0

In the project I am currently employed to we need the feature of generating Base64QRCode for different image types. And recently we added linux support. To still use this library we had to re-implement these classes by ourselves and with help of https://github.com/JPlenert/QRCoder-ImageSharp. Unfortunately we couldn't use the mentioned fork directly because of some internal concerns.

Breaking changes

ImageType enum was moved into the separate file from below the Base64QRCode class. Otherwise, changes were made backwar-compatible. Only new classes were added, old ones were left as is.

Test plan

Build QRCoderConsole project and go to the bin folder. Execute following command in net45, net5.0, net5.0-windows, net6.0-windows and compare the results.
.\QRCoderConsole.exe -p "Input text" -e Q -s 20 -o file.png

Following tool can be used: https://www.diffchecker.com/image-compare/. net45 needs to be compared with all the others.

[Shane32]

I wouldn't endorse adding a third party (non-MS) dependency to the base QRCoder NuGet package. I would rather see something like PR #407 where there are separate packages depending on the needs of the user.