Skip variant selectors in deleteCharBackward

FIX: `deleteCharBackward` will no longer delete variant selector characters
as separate characters.

See https://discuss.codemirror.net/t/backspace-on-some-multi-codepoint-glyphs-deletes-characters-separately/7403

src/commands.ts

@@ -478,6 +478,8 @@ const deleteByChar = (target: CommandTarget, forward: boolean) => deleteBy(targe
     targetPos = findClusterBreak(line.text, pos - line.from, forward, forward) + line.from
     if (targetPos == pos && line.number != (forward ? state.doc.lines : 1))
       targetPos += forward ? 1 : -1
+    else if (!forward && /[\ufe00-\ufe0f]/.test(line.text.slice(targetPos - line.from, pos - line.from)))
+      targetPos = findClusterBreak(line.text, targetPos - line.from, false, false)
   }
   return targetPos
 })