Consider adding [Legacy​ASCII​Case​Insentive​Enum] for element​.insertAdjacent* APIs

[ExE-Boss]

The element.insertAdjacentElement, element.insertAdjacentText and element.insertAdjacentHTML APIs take a where/position argument of DOMString that is ASCII lowercased and treated as if it were converted to the following enum:

enum InsertPosition {
	"beforebegin",
	"afterbegin",

	"beforeend",
	"afterend"
};

Except that instead of throwing a TypeError or a RangeError ^#489, a "SyntaxError" DOMException is thrown instead.

See also:

• Use RangeError rather than TypeError for enum mismatch #489
• Can we have a way for enums to not throw if there are unknown values #491
• Enums that does not throw? #893

[annevk]

I think there should be more precedent before we standardize upon a pattern in IDL. Unless there's a reason handling it prose is difficult, but that does not appear to be the case here?