Fix `Symbol` and `BigInt` constructors (#2032)

It changes the following:
- `Symbol` object has `[[Construct]]` internal method
- Fix and comment step 1 in `Symbol` constructor.
- Implement step 1 in `BigInt` constructor.

boa_engine/src/builtins/bigint/mod.rs

@@ -46,12 +46,12 @@ impl BuiltIn for BigInt {
         )
         .name(Self::NAME)
         .length(Self::LENGTH)
+        .callable(true)
+        .constructor(true)
         .method(Self::to_string, "toString", 0)
         .method(Self::value_of, "valueOf", 0)
         .static_method(Self::as_int_n, "asIntN", 2)
         .static_method(Self::as_uint_n, "asUintN", 2)
-        .callable(true)
-        .constructor(true)
         .property(
             to_string_tag,
             Self::NAME,
@@ -77,7 +77,16 @@ impl BigInt {
     ///
     /// [spec]: https://tc39.es/ecma262/#sec-bigint-objects
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt/BigInt
-    fn constructor(_: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
+    fn constructor(
+        new_target: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        // 1. If NewTarget is not undefined, throw a TypeError exception.
+        if !new_target.is_undefined() {
+            return context.throw_type_error("BigInt is not a constructor");
+        }
+
         let value = args.get_or_undefined(0);
 
         // 2. Let prim be ? ToPrimitive(value, number).

boa_engine/src/builtins/symbol/mod.rs

@@ -112,6 +112,8 @@ impl BuiltIn for Symbol {
         )
         .name(Self::NAME)
         .length(Self::LENGTH)
+        .callable(true)
+        .constructor(true)
         .static_method(Self::for_, "for", 1)
         .static_method(Self::key_for, "keyFor", 1)
         .static_property("asyncIterator", symbol_async_iterator, attribute)
@@ -135,8 +137,6 @@ impl BuiltIn for Symbol {
             None,
             Attribute::CONFIGURABLE | Attribute::NON_ENUMERABLE,
         )
-        .callable(true)
-        .constructor(false)
         .property(
             symbol_to_string_tag,
             Self::NAME,
@@ -173,14 +173,19 @@ impl Symbol {
         args: &[JsValue],
         context: &mut Context,
     ) -> JsResult<JsValue> {
-        if new_target.is_undefined() {
+        // 1. If NewTarget is not undefined, throw a TypeError exception.
+        if !new_target.is_undefined() {
             return context.throw_type_error("Symbol is not a constructor");
         }
+
+        // 2. If description is undefined, let descString be undefined.
+        // 3. Else, let descString be ? ToString(description).
         let description = match args.get(0) {
             Some(value) if !value.is_undefined() => Some(value.to_string(context)?),
             _ => None,
         };
 
+        // 4. Return a new unique Symbol value whose [[Description]] value is descString.
         Ok(JsSymbol::new(description).into())
     }