LibJS: Replace a DeprecatedString with String

Refactors Date class to use String instead of DeprecatedString.

Changes use of the Date class in DatePrototype accordingly.

Userland/Libraries/LibJS/Runtime/Date.cpp

@@ -34,7 +34,7 @@ Date::Date(double date_value, Object& prototype)
 {
 }
 
-DeprecatedString Date::iso_date_string() const
+ErrorOr<String> Date::iso_date_string() const
 {
     int year = year_from_time(m_date_value);
 
@@ -59,7 +59,7 @@ DeprecatedString Date::iso_date_string() const
     builder.appendff("{:03}", ms_from_time(m_date_value));
     builder.append('Z');
 
-    return builder.to_deprecated_string();
+    return builder.to_string();
 }
 
 // DayWithinYear(t), https://tc39.es/ecma262/#eqn-DayWithinYear

Userland/Libraries/LibJS/Runtime/Date.h

@@ -23,7 +23,7 @@ public:
     double date_value() const { return m_date_value; }
     void set_date_value(double value) { m_date_value = value; }
 
-    DeprecatedString iso_date_string() const;
+    ErrorOr<String> iso_date_string() const;
 
 private:
     Date(double date_value, Object& prototype);

Userland/Libraries/LibJS/Runtime/DatePrototype.cpp

@@ -967,7 +967,7 @@ JS_DEFINE_NATIVE_FUNCTION(DatePrototype::to_iso_string)
     if (!Value(this_object->date_value()).is_finite_number())
         return vm.throw_completion<RangeError>(ErrorType::InvalidTimeValue);
 
-    auto string = this_object->iso_date_string();
+    auto string = TRY_OR_THROW_OOM(vm, this_object->iso_date_string());
     return PrimitiveString::create(vm, move(string));
 }