[Design] Add ValidationMessage TagHelper.

#1250
Nope, I was confused and `ContentBehavior.Append` is wrong. There's two approaches for this element and I forget what we decided (didn't save any comments about this point in the sample):
1. `ContentBehavior.Replace`, automatically throwing away whatever the user enters between the start and end tags
2. `ContentBehavior.Modify` and then special-case `Content` which is not whitespace

If we chose (2), note the current approach won't work correctly because `Generator.GenerateValidationMessage()` special-cases `null` or empty, not whitespace. that method will also (re-)HTML-escape what the user typed in the Razor file. It's never possible to read the original `Content` and use it as if it's a regular `string`.
so if we chose (2), just ignore the `tagBuilder.InnerHtml` if the original `Content` is not whitespace. override the user's whitespace otherwise.