Refactor firstchar function for conciseness and modern syntax

- Replaced var with const for improved variable management
- Used optional chaining for a more concise return statement
- Enhanced readability while preserving functionality

lib/types/json.js

@@ -189,11 +189,8 @@ function createStrictSyntaxError (str, char) {
  */
 
 function firstchar (str) {
-  var match = FIRST_CHAR_REGEXP.exec(str)
-
-  return match
-    ? match[1]
-    : undefined
+  const match = FIRST_CHAR_REGEXP.exec(str)
+  return match?.[1]
 }
 
 /**