…<string> as it allows for less

it allows us for creating the host and everything else after parsing has finished
UseCommandHandler must specify the handler for instance of a Command, not per type as there can be multiple commands of the same type

# Conflicts:
#	src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt
#	src/System.CommandLine.Tests/Invocation/InvocationPipelineTests.cs
#	src/System.CommandLine/Builder/CommandLineBuilder.cs
#	src/System.CommandLine/Builder/CommandLineBuilderExtensions.cs
#	src/System.CommandLine/CommandLineConfiguration.cs

# Conflicts:
#	src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt
#	src/System.CommandLine.Generator.Tests/GeneratedCommandHandlerTests.cs
#	src/System.CommandLine.Hosting/HostingAction.cs
#	src/System.CommandLine.NamingConventionBinder/ServiceProvider.cs
#	src/System.CommandLine.Tests/CommandExtensionsTests.cs
#	src/System.CommandLine.Tests/Help/HelpBuilderTests.Customization.cs
#	src/System.CommandLine.Tests/Invocation/InvocationPipelineTests.cs
#	src/System.CommandLine.Tests/UseExceptionHandlerTests.cs
#	src/System.CommandLine.Tests/UseHelpTests.cs
#	src/System.CommandLine/Help/HelpOption.cs
#	src/System.CommandLine/Help/HelpOptionAction.cs
#	src/System.CommandLine/Invocation/InvocationContext.cs