LibJS: Implement constructor/non-constructor function calls

This adds Function::construct() for constructor function calls via `new`
keyword. NativeFunction doesn't have constructor behaviour by default,
ScriptFunction simply calls call() in construct()

Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -46,6 +46,14 @@ DateConstructor::~DateConstructor()
 }
 
 Value DateConstructor::call(Interpreter& interpreter)
+{
+    auto* date = static_cast<Date*>(construct(interpreter).as_object());
+    if (!date)
+        return {};
+    return js_string(interpreter.heap(), date->string());
+}
+
+Value DateConstructor::construct(Interpreter& interpreter)
 {
     // TODO: Support args
     struct timeval tv;

Libraries/LibJS/Runtime/DateConstructor.h

@@ -36,8 +36,10 @@ public:
     virtual ~DateConstructor() override;
 
     virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
 
 private:
+    virtual bool has_constructor() const override { return true; }
     virtual const char* class_name() const override { return "DateConstructor"; }
 
     static Value now(Interpreter&);

Libraries/LibJS/Runtime/Function.h

@@ -36,6 +36,7 @@ public:
     virtual ~Function();
 
     virtual Value call(Interpreter&) = 0;
+    virtual Value construct(Interpreter&) = 0;
 
 protected:
     Function();

Libraries/LibJS/Runtime/NativeFunction.cpp

@@ -44,4 +44,9 @@ Value NativeFunction::call(Interpreter& interpreter)
     return m_native_function(interpreter);
 }
 
+Value NativeFunction::construct(Interpreter&)
+{
+    return {};
+}
+
 }

Libraries/LibJS/Runtime/NativeFunction.h

@@ -37,6 +37,9 @@ public:
     virtual ~NativeFunction() override;
 
     virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
+
+    virtual bool has_constructor() const { return false; }
 
 protected:
     NativeFunction() {}

Libraries/LibJS/Runtime/ObjectConstructor.cpp

@@ -50,6 +50,11 @@ Value ObjectConstructor::call(Interpreter& interpreter)
     return interpreter.heap().allocate<Object>();
 }
 
+Value ObjectConstructor::construct(Interpreter& interpreter)
+{
+    return call(interpreter);
+}
+
 Value ObjectConstructor::get_own_property_names(Interpreter& interpreter)
 {
     if (interpreter.call_frame().arguments.size() < 1)

Libraries/LibJS/Runtime/ObjectConstructor.h

@@ -36,8 +36,10 @@ public:
     virtual ~ObjectConstructor() override;
 
     virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
 
 private:
+    virtual bool has_constructor() const override { return true; }
     virtual const char* class_name() const override { return "ObjectConstructor"; }
 
     static Value get_own_property_names(Interpreter&);

Libraries/LibJS/Runtime/ScriptFunction.cpp

@@ -55,4 +55,9 @@ Value ScriptFunction::call(Interpreter& interpreter)
     return interpreter.run(m_body, arguments, ScopeType::Function);
 }
 
+Value ScriptFunction::construct(Interpreter& interpreter)
+{
+    return call(interpreter);
+}
+
 }

Libraries/LibJS/Runtime/ScriptFunction.h

@@ -39,6 +39,7 @@ public:
     const Vector<FlyString>& parameters() const { return m_parameters; };
 
     virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
 
 private:
     virtual bool is_script_function() const final { return true; }