LibJS: Convert this_bigint_value() to ThrowCompletionOr

Author: https://github.com/linusg
Commit: f6cf44c3db
Pull-request: https://github.com/SerenityOS/serenity/pull/10533
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/BigIntObject.h

@@ -20,7 +20,9 @@ public:
     BigIntObject(BigInt&, Object& prototype);
     virtual ~BigIntObject();
 
-    const BigInt& bigint() const { return m_bigint; }
+    BigInt const& bigint() const { return m_bigint; }
+    BigInt& bigint() { return m_bigint; }
+
     virtual Value value_of() const override
     {
         return Value(&m_bigint);