Enable nullable reference types for Hosting

[WeihanLi]

Fixes #1760

• Enable nullable reference types for Hosting
• Renaming commandline => commandLine to fix typo
• Remove unnecessary ToList for configuration extension

[jonsequitur]

Why this change? I would think the IHost is a required service here.

[WeihanLi]

GetRequiredService would return service and ensure the service is not null, GetService would return null when no service found, think we expect IHost exists, so maybe better to use GetRequiredService
https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.serviceproviderserviceextensions.getrequiredservice?view=dotnet-plat-ext-6.0

[jonsequitur]

This raises a question about whether callers can expect this to have been configured or not. I suspect the answer is that they can and if it's not available, it could be because of a code defect.

[WeihanLi]

Think it should not be registered when IHost or the handler is null, maybe I could add a null check for that

[jonsequitur]

I would think we should not allow null here.

[WeihanLi]

I think so before, from test code

command-line-api/src/System.CommandLine.Hosting.Tests/HostingTests.cs

Lines 401 to 403 in 209b724

	IHost host = invCtx.GetHost();
	host.Should().BeNull();
	hostAsserted = true;

it may be null, if this should not be null, guess we should throw an InvalidOperationException?