Update schema for ImplicitUsings feature

[DamianEdwards]

As per dotnet/sdk#19521, update schema info in src/MSBuild/MSBuild/Microsoft.Build.CommonTypes.xsd for property and item type introduced in .NET 6 for the implicit usings feature:

• <ImplicitUsings> property, equal to True, true, or enable
• <Using Include="" Alias="" Static="true" /> item type where:
  • Definition value is the namespace or type identifier to add, e.g. Microsoft.AspNetCore
  • Alias attribute is the alias identifier to use to identify the type being aliased
  • Static is a boolean indicating whether the identifier should be registered as a static import