LibJS: Store basic traceback in Exception

Nothing fancy like line numbers, but Exception now stores a list of
function names up to the current call frame.

Libraries/LibJS/Runtime/Exception.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/Exception.h>
 
 namespace JS {
@@ -31,6 +32,13 @@ namespace JS {
 Exception::Exception(Value value)
     : m_value(value)
 {
+    auto call_stack = interpreter().call_stack();
+    for (ssize_t i = call_stack.size() - 1; i >= 0; --i) {
+        auto function_name = call_stack[i].function_name;
+        if (function_name.is_empty())
+            function_name = "<anonymous>";
+        m_trace.append(function_name);
+    }
 }
 
 Exception::~Exception()

Libraries/LibJS/Runtime/Exception.h

@@ -37,12 +37,14 @@ public:
     virtual ~Exception() override;
 
     Value value() const { return m_value; }
+    const Vector<String>& trace() const { return m_trace; }
 
 private:
     virtual const char* class_name() const override { return "Exception"; }
     virtual void visit_children(Visitor&) override;
 
     Value m_value;
+    Vector<String> m_trace;
 };
 
 }