CSSUtils.findSelectorAtDocumentPos() matching in embedded stylesheet

`CSSUtils.findSelectorAtDocumentPos()` does not correctly match *the first selector* in an embedded stylesheet. It spills out of the `<style>` tag and incorrectly matches everything up to the start of the document.

This PR includes a test to demonstrate this behavior.

I want to fix this problem. My idea is to check for the mode (css | text/x-scss | text/x-less) while looping tokens at: https://github.com/adobe/brackets/blob/master/src/language/CSSUtils.js#L1087

Is this a correct approach? Do you have another suggestion?