Add setSelectionRange to HTMLTextAreaElement
Add setSelectionRange to HTMLTextAreaElement

Summary: According to the [[http://www.w3.org/TR/html5/forms.html#dom-textarea/input-setselectionrange | W3 Spec]]/[[https://developer.mozilla.org/en-US/docs/Web/API/HTMLTextAreaElement | MDN]], `setSelectionRange` is
a method on `HTMLTextAreaElement`s. This came up while flow-ifying
`InlinePostEditor`, and it seems like a straightforward fix.

There's an optional third argument for this function that wasn't initially
specified, so I've also added that. I made a new type alias for it, but if I
should be using literals or some other convention, I'm happy to change it.

Reviewed By: mroch

Differential Revision: D2680427

fb-gh-sync-id: 2813b77236b34eefd26e574a0361006e19286a83

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