Stop storing project intermediates in the assets directory

This merge request fixes an issue with the .NET project generator where generated project files will pollute the user's assets directory. This would then result in the engine trying to import random MSBuild trash as game assets, impacting startup times and flooding the console with failed import warnings.

All .csproj files for the project now sit in the project root, next to the generated solution file.

A Directory.Build.props file is generated that forces MSBuild to move the obj directory (intermediate build files and IDE crap) to .toolbox, which belongs in .gitignore.

.NET's implicit project file importing feature has been disabled in the generated C# projects. Instead, the engine figures out which directories belong to the project, and includes them as <Compile> items. Directories from other projects will be excluded. This is similar to Unity's behaviour, but plays better with modern SDK-style projects.