Add WeakRef implementation

1. Refactor JSObject weak reference list to support different kind of weak reference
2. Support Symbol as WeakMap key

doc/quickjs.texi

@@ -281,9 +281,7 @@ The following features are not supported yet:
 
 @item Tail calls@footnote{We believe the current specification of tails calls is too complicated and presents limited practical interests.}
 
-@item WeakRef and FinalizationRegistry objects
-
-@item Symbols as WeakMap keys
+@item FinalizationRegistry objects
 
 @end itemize
 

qjsc.c

@@ -77,6 +77,7 @@ static const FeatureEntry feature_list[] = {
 #define FE_MODULE_LOADER 9
     { "module-loader", NULL },
     { "bigint", "BigInt" },
+    { "weakref", "WeakRef" },
 };
 
 void namelist_add(namelist_t *lp, const char *name, const char *short_name,

quickjs-atom.h

@@ -269,5 +269,6 @@ DEF(Symbol_asyncIterator, "Symbol.asyncIterator")
 #ifdef CONFIG_BIGNUM
 DEF(Symbol_operatorSet, "Symbol.operatorSet")
 #endif
+DEF(WeakRef, "WeakRef")
 
 #endif /* DEF */