Rename `TagHelperContent.Append(IHtmlContent)` overload to be `TagHelperContent.AppendHtml(IHtmlContent)`

[NTaylorMullen]

@DamianEdwards and I discussed the current implementation of TagHelperContent.Append(IHtmlContent htmlContent) and felt that it makes more sense to append an IHtmlContent via an AppendHtml method rather than an overload to Append. In TagHelperContents case we already have an AppendHtml helper method that wraps strings to ensure they aren't encoded. We should just overload its existing AppendHtml to take an IHtmlContent and remove the current Append(IHtmlContent) overload.

It's already decently confusing when determining how to append encoded/unencoded content. This would help make it clearer since IHtmlContent is HTML.

[dougbu]

The language here is never going to be quite right. An IHtmlContent implementation may or may not be pre-encoded. We're not talking about just HtmlString.

[DamianEdwards]

True, but surely IHtmlContent represents, you know, content that is HTML 😄

I think the rename makes sense here.

[dougbu]

@DamianEdwards exactly. User already knows it's IHtmlContent. So why repeat that information in the IHtmlContentBuilder (aspnet/HtmlAbstractions#4) or TagHelperContent method name?

[DamianEdwards]

Because when I tried to use it I didn't even see the overload of Append that takes IHtmlContent, given that we have a method called AppendHtml. I ended up calling AppendHtml(myHtmlContent.ToString()) which is a pit of failure (I got encoded HTML, not at all what I wanted). My thinking was "I have something that knows it's HTML, I want to append it as HTML, so call AppendHtml".