[8.x] Add htmlString helper method

[Larsklopstra]

Motivation:

I often use the HtmlString to render Stringable data without using {!! $content !!}.

Before:

return new HtmlString(Str::of($content)->markdown());

return new HtmlString(Str::markdown($content));

After:

return Str::of($content)
    ->markdown()
    ->html();

[driesvints]

@selcukcukur instead of downvoting please provide feedback why you don't like this addition.

[driesvints]

@Larsklopstra maybe just ->html()?