v0.7.0-pre-20211217.15

Changes:

• Dropped support for netcoreapp2.1 as it's out of support
• Updated dependency on Markdig to 0.26.0
• Added SourceLink
• Added IntelliSense documentation file
• Documentation site now runs on .NET 6.0 on Linux

Available on NuGet:

• https://www.nuget.org/packages/TagHelperPack/0.7.0-pre-20211217.15