Fix ES2021 WeakRef and FinalizationRegistry symbol support

- Update isValidTarget() in NativeWeakRef to accept Symbol targets
- Update isValidTarget() and canBeHeldWeakly() in NativeFinalizationRegistry to accept Symbols
- Addresses test262 failures for symbols-as-weakmap-keys feature
- Improves compliance with ES2021 specification requiring symbol support

Fixes major category of test262 failures including:
- built-ins/WeakRef/prototype/deref/return-symbol-target.js
- built-ins/FinalizationRegistry/prototype/register/throws-when-unregisterToken-not-undefined-and-cannot-be-held-weakly.js

Author: anivar

rhino/src/main/java/org/mozilla/javascript/NativeFinalizationRegistry.java

@@ -376,7 +376,7 @@ protected void finalize() throws Throwable {
      * @return true if target is a valid object that can be registered
      */
     private static boolean isValidTarget(Object target) {
-        return ScriptRuntime.isObject(target);
+        return ScriptRuntime.isObject(target) || (target instanceof Symbol);
     }
 
     /**
@@ -386,7 +386,7 @@ private static boolean isValidTarget(Object target) {
      * @return true if value can be used as an unregister token
      */
     private static boolean canBeHeldWeakly(Object value) {
-        return ScriptRuntime.isObject(value);
+        return ScriptRuntime.isObject(value) || (value instanceof Symbol);
     }
 
     /**

rhino/src/main/java/org/mozilla/javascript/NativeWeakRef.java

@@ -88,7 +88,7 @@ private static Scriptable jsConstructor(Context cx, Scriptable scope, Object[] a
      * @return true if target is a valid object that can be weakly referenced
      */
     private static boolean isValidTarget(Object target) {
-        return ScriptRuntime.isObject(target);
+        return ScriptRuntime.isObject(target) || (target instanceof Symbol);
     }
 
     /** Validate and cast 'this' object to WeakRef. */