TagHelperContext.GetChildContentAsync moved to TagHelperOutput

[NTaylorMullen]

Due to the new TagHelper addition of Init used to setup your TagHelperContext and other related item bag info for executed children to consume we decided that the ability to execute children doesn't entirely make sense on the context object since it has no connection to the actual structure of the element.

The alternative would have been to make GetChildContentAsync throw when invoked within the Init method (yuck).

Before

public class CustomTagHelper : TagHelper
{
    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        await context.GetChildContentAsync();
    }
}

After

public class CustomTagHelper : TagHelper
{
    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        await output.GetChildContentAsync();
    }
}

[NTaylorMullen]

For discussion and feedback, please see the related discussion issue aspnet/Razor#583.