Add TagHelperSample.Web

Add Tag Helper sample, #1242
w.r.t to the second point, I'm using all of the helpers I'm aware of. I'm not using some variants of `<input/>` (checkbox, password, or radio) nor all `<a/>` and `<form/>` bound attributes. but all of that follows the patters shown here. what exactly (other than the `<form/>` point you mentioned elsewhere) do you think is missing?