feat(web): proper Unicode emoji handling

> https://github.com/keymanapp/keyman/blob/3b902369efa999e22fcea00d00f27d66487e2853/ios/engine/KMEI/KeymanEngine/Classes/InputViewController.swift#L376-L379
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> We should certainly have a TODO issue to properly support deletion of emoji...
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> _Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/4938#discussion_r618037572_

KeymanWeb is currently unable to backspace entire emoji clusters (where one emoji is represented by multiple code points) in the same way that iOS's default 'text document proxy' can with its `deleteBackward` method. As a result, our mobile app engines can only do piecemeal deletions of such characters right now.

This also holds true for Android, though at least that engine didn't need extra handling in order to enforce context synchronization as part of #4938.