feat: Add ImplicitUsings and Nullable to default game templates

Is your feature request related to a problem? Please describe.

The new game templates should have in MyGame.Game.csproj this included, which is default for .NET 10.

<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>

Describe the solution you'd like The templates should automatically add

<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>

Describe alternatives you've considered Adding it manually.

Additional context These settings are good practises:

• ImplicitUsings: eliminates repetetive namespace imports
• Nullable: Catches NullReferenceException bugs at a compile time