Show warning when non-void element has an end tag

[dougbu]

We have had a few issues filed around the .cshtml editor accepting non-void HTML elements without a separate end tag e.g. <select /> versus <select></select> (see aspnet/Mvc#4058) or <textarea/> versus <textarea></textarea> (see aspnet/Mvc#3964). The most common case is that an MVC tag helper targeting the element does not generate working HTML.

Anything we can do better to show .cshtml authors what they're writing is invalid would improve the user experience.

[barrytang]

Tracked by TFS#189952 internally.

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