[dom] Text nodes .closest (#161)

Text nodes have no .closest method, when working with Range, I find useful to do `Text.prototype.closest = function(s) {return this.parentNode.closest(s); } //with this.parentNode&& if we want to be strict` 

For me it makes sense to be able to search from a textnode, .matches wouldn't make sense, but .closest would

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https://github.com/whatwg/dom/issues/161