R1:fe042def42b0

Add a Javascript method to find the pixel position of a range in a textarea

Summary:
Ref T3725. This might eventually allow us to do `@username` typeaheads in textareas.

Javascript!!!

Test Plan:
Dumped this into console and got a "<<<" at the caret position in Safari, Firefox and Chrome.

```
setInterval(function() {

var area = JX.$('comment-content');
var r = JX.TextAreaUtils.getSelectionRange(area);
var d = JX.TextAreaUtils.getPixelDimensions(area, r.start, r.end);

JX.log(d);

try {
JX.DOM.remove…