* Get local builds working
- no solution file or build scripts but `dotnet pack` does the trick
- last part of #32027 unless we need Arcade or a CI build urgently
Main work was to add a few missing files and modify them for this environment
- mostly copied from dotnet/aspnetcore
- Directory.Build.props and Directory.Build.targets created from scratch
- some content copied from dotnet/arcade and dotnet/aspnetcore
- NuGet.config and eng/Versions.props copied but pared down (a lot) to mandatory bits for an isolated build
- new eng/ files mostly came from src/ProjectTemplates/ in dotnet/aspnetcore
- eng/DotNetPackageIcon.png came from src/Microsoft.DotNet.Arcade.Sdk/tools/Assets/ in dotnet/arcade
- eng/Packaging.targets copied from src/Microsoft.DotNet.Arcade.Sdk/tools/Workarounds.targets in dotnet/arcade
nits:
- fix whitespace Directory.Build.targets target added to `$(Description)`
- fix typo in the `_AppendRepositoryUrlToPackageDescription` target name
- remove src/.gitignore and move content into new root .gitignore file
- remove SECURITY.md trailing whitespace
