Insert warning on deprecated / obsolete HTML Elements

[lannonbr]

As a feature that could be implemented after #22, It would be neat to either display a warning / error on html elements which are deprecated (blink, marquee, etc.).

In MDN's browser-compat-data repo, html elements have a deprecated property on them (For instance, here on the blink tag, so if the language server switches to use MDN's data, this could be a fairly quick feature to implement.

[octref]

Currently we only have this inside our code:

But MDN does have relevant info, for example https://developer.mozilla.org/en-US/docs/Web/HTML/Element/menuitem. After finishing up #44 I can look into this.

[aeschli]

Validation is not planed.