LibJS: Implement Temporal.Instant.prototype.epochNanoseconds

Author: https://github.com/linusg
Commit: 117323f2d9
Pull-request: https://github.com/SerenityOS/serenity/pull/8568
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/bgianfo ✅

Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.epochNanoseconds.js

@@ -0,0 +1,25 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        expect(new Temporal.Instant(0n).epochNanoseconds).toBe(0n);
+        expect(new Temporal.Instant(1n).epochNanoseconds).toBe(1n);
+        expect(new Temporal.Instant(999n).epochNanoseconds).toBe(999n);
+        expect(new Temporal.Instant(8_640_000_000_000_000_000_000n).epochNanoseconds).toBe(
+            8_640_000_000_000_000_000_000n
+        );
+
+        expect(new Temporal.Instant(-0n).epochNanoseconds).toBe(-0n);
+        expect(new Temporal.Instant(-1n).epochNanoseconds).toBe(-1n);
+        expect(new Temporal.Instant(-999n).epochNanoseconds).toBe(-999n);
+        expect(new Temporal.Instant(-8_640_000_000_000_000_000_000n).epochNanoseconds).toBe(
+            -8_640_000_000_000_000_000_000n
+        );
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.Instant object", () => {
+        expect(() => {
+            Reflect.get(Temporal.Instant.prototype, "epochNanoseconds", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Instant");
+    });
+});