Only add global usings when ImplicitUsings is enabled

[ronaldbarendse]

Prerequisites

• I have added steps to test this contribution in the description below

Description

As suggested by @JimBobSquarePants, the global usings in the Umbraco.Cms package should only be added when ImplicitUsings are enabled in the consuming project.

Because users could have disabled ImplicitUsings on their v10 project, but still depend on the global usings Umbraco adds (and therefore not have added explicit usings), this might be a source code breaking change. I've still targeted this PR to v10, so we can include it in the next minor version or otherwise easily cherry-pick into v11.

Testing can be done by first disabling implicit usings in the Umbraco.Web.UI (before applying this PR) by adding <ImplicitUsings>false</ImplicitUsings> to the project file and adding explicit usings for the System.* and Microsoft.* namespaces (see docs).

Next apply this PR and notice you now also have to add explicit usings for Umbraco.Cms.Core.DependencyInjection and Umbraco.Extensions 👍🏻

[ronaldbarendse]

Since this change potentially results in a code breaking change, I've targeted v11 and created a new PR: #13491.