Emoji: Fix TextEditor to properly delete emoji with backspace/delete

[ftommasi]

Added some checks to the Backspace and Delete Key event handlers in TextEditor to look for Emoji in the previous/next word respectively (Span until prev/next whitespace). If an emoji is found, then the number of code points for that emoji is deleted. If no emoji is found, then it is assumed that there are no emoji and a single codepoint is deleted.

Fix #15261

(This is a repost of #15515 I accidentally closed my branch trying to work on a separate issue)