LibJS: Add Temporal.Instant.prototype.round()

As well as the required Abstract Operations.

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

@@ -0,0 +1,62 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const instant = new Temporal.Instant(1111111111111n);
+        expect(instant.round({ smallestUnit: "second" }).epochNanoseconds).toBe(1111000000000n);
+        expect(
+            instant.round({ smallestUnit: "second", roundingMode: "ceil" }).epochNanoseconds
+        ).toBe(1112000000000n);
+        expect(
+            instant.round({ smallestUnit: "minute", roundingIncrement: 30, roundingMode: "floor" })
+                .epochNanoseconds
+        ).toBe(0n);
+        expect(
+            instant.round({
+                smallestUnit: "minute",
+                roundingIncrement: 30,
+                roundingMode: "halfExpand",
+            }).epochNanoseconds
+        ).toBe(1800000000000n);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.Instant object", () => {
+        expect(() => {
+            Temporal.Instant.prototype.round.call("foo", {});
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Instant");
+    });
+
+    test("invalid rounding mode", () => {
+        expect(() => {
+            const instant = new Temporal.Instant(1n);
+            instant.round({ smallestUnit: "second", roundingMode: "serenityOS" });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option roundingMode");
+    });
+
+    test("invalid smallest unit", () => {
+        expect(() => {
+            const instant = new Temporal.Instant(1n);
+            instant.round({ smallestUnit: "serenityOS" });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option smallestUnit");
+    });
+
+    test("increment may not be NaN", () => {
+        expect(() => {
+            const instant = new Temporal.Instant(1n);
+            instant.round({ smallestUnit: "second", roundingIncrement: NaN });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option roundingIncrement");
+    });
+
+    test("increment may smaller than 1 or larger than maximum", () => {
+        const instant = new Temporal.Instant(1n);
+        expect(() => {
+            instant.round({ smallestUnit: "second", roundingIncrement: -1 });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option roundingIncrement");
+        expect(() => {
+            instant.round({ smallestUnit: "second", roundingIncrement: 0 });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option roundingIncrement");
+        expect(() => {
+            instant.round({ smallestUnit: "second", roundingIncrement: Infinity });
+        }).toThrowWithMessage(RangeError, "is not a valid value for option roundingIncrement");
+    });
+});