[FIX] web_editor: delete unicode character

Before this commit, when trying to delete one
unicode character (e.g. 😍), only one javascript character was delete
but one unicode character (e.g. 😍) could be two javascript characters
as the javascript string is coded in utf-16.

This commit fixes that by calculating the size of one unicode character
from the javascript string.

Task-2658890

closes odoo/odoo#79071

X-original-commit: fea5b87615077f5f41dec8c4d1480f157bbd26ae
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/lib/odoo-editor/src/commands/deleteBackward.js

@@ -35,9 +35,11 @@ Text.prototype.oDeleteBackward = function (offset, alreadyMoved = false) {
         return;
     }
 
-    // First, split around the character where the backspace occurs
-    const firstSplitOffset = splitTextNode(this, offset - 1);
-    const secondSplitOffset = splitTextNode(parentNode.childNodes[firstSplitOffset], 1);
+    // Get the size of the unicode character to remove.
+    const charSize = [...this.nodeValue.slice(0, offset)].pop().length;
+    // Split around the character where the backspace occurs.
+    const firstSplitOffset = splitTextNode(this, offset - charSize);
+    const secondSplitOffset = splitTextNode(parentNode.childNodes[firstSplitOffset], charSize);
     const middleNode = parentNode.childNodes[firstSplitOffset];
 
     // Do remove the character, then restore the state of the surrounding parts.