Emoji: Add some extra IE11 compatibility.

IE 11's implementation of MutationObserver is buggy. It unnecessarily splits text nodes when it encounters a HTML template interpolation symbol ( "{{", for example ). So, we join the text nodes back together as a work-around.

Fixes #35977 for trunk.


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git-svn-id: https://core.svn.wordpress.org/trunk@36784 1a063a9b-81f0-0310-95a4-ce76da25c4cd