LibJS: Start implementing Temporal.Instant

Just like the initial Temporal.TimeZone commit, this patch adds the
Instant object itself, its constructor and prototype (currently empty),
and two required abstract operations.

Userland/Libraries/LibJS/Runtime/Temporal/InstantConstructor.cpp

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/Instant.h>
+#include <LibJS/Runtime/Temporal/InstantConstructor.h>
+
+namespace JS::Temporal {
+
+// 8.1 The Temporal.Instant Constructor, https://tc39.es/proposal-temporal/#sec-temporal-instant-constructor
+InstantConstructor::InstantConstructor(GlobalObject& global_object)
+    : NativeFunction(vm().names.Instant.as_string(), *global_object.function_prototype())
+{
+}
+
+void InstantConstructor::initialize(GlobalObject& global_object)
+{
+    NativeFunction::initialize(global_object);
+
+    auto& vm = this->vm();
+
+    // 8.2.1 Temporal.Instant.prototype, https://tc39.es/proposal-temporal/#sec-temporal-instant-prototype
+    define_direct_property(vm.names.prototype, global_object.temporal_instant_prototype(), 0);
+
+    define_direct_property(vm.names.length, Value(1), Attribute::Configurable);
+}
+
+// 8.1.1 Temporal.Instant ( epochNanoseconds ), https://tc39.es/proposal-temporal/#sec-temporal.instant
+Value InstantConstructor::call()
+{
+    auto& vm = this->vm();
+
+    // 1. If NewTarget is undefined, then
+    // a. Throw a TypeError exception.
+    vm.throw_exception<TypeError>(global_object(), ErrorType::ConstructorWithoutNew, "Temporal.Instant");
+    return {};
+}
+
+// 8.1.1 Temporal.Instant ( epochNanoseconds ), https://tc39.es/proposal-temporal/#sec-temporal.instant
+Value InstantConstructor::construct(FunctionObject& new_target)
+{
+    auto& vm = this->vm();
+    auto& global_object = this->global_object();
+
+    // 2. Let epochNanoseconds be ? ToBigInt(epochNanoseconds).
+    auto* epoch_nanoseconds = vm.argument(0).to_bigint(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. If ! IsValidEpochNanoseconds(epochNanoseconds) is false, throw a RangeError exception.
+    if (!is_valid_epoch_nanoseconds(*epoch_nanoseconds)) {
+        vm.throw_exception<RangeError>(global_object, ErrorType::TemporalInvalidEpochNanoseconds);
+        return {};
+    }
+
+    // 4. Return ? CreateTemporalInstant(epochNanoseconds, NewTarget).
+    return create_temporal_instant(global_object, *epoch_nanoseconds, &new_target);
+}
+
+}