fix prompt-bracket-checker miscounting of literal tokens (#16669)

Author: w-e-w

extensions-builtin/prompt-bracket-checker/javascript/prompt-bracket-checker.js

@@ -5,7 +5,7 @@
 
 function checkBrackets(textArea, counterElt) {
     var counts = {};
-    (textArea.value.match(/[(){}[\]]/g) || []).forEach(bracket => {
+    (textArea.value.match(/(?<!\\)[(){}[\]]/g) || []).forEach(bracket => {
         counts[bracket] = (counts[bracket] || 0) + 1;
     });
     var errors = [];