LibJS: Implement the Temporal.Instant constructor

And the simple Temporal.Instant.prototype getters, so that the
constructed Temporal.Instant may actually be validated.

Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.from.js

@@ -0,0 +1,64 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Instant.from).toHaveLength(1);
+    });
+
+    test("Instant instance argument", () => {
+        const instant = new Temporal.Instant(123n);
+        expect(Temporal.Instant.from(instant).epochNanoseconds).toBe(123n);
+    });
+
+    test("Instant string argument", () => {
+        expect(Temporal.Instant.from("1975-02-02T14:25:36.123456789Z").epochNanoseconds).toBe(
+            160583136123456789n
+        );
+        // Time zone is not validated
+        expect(
+            Temporal.Instant.from("1975-02-02T14:25:36.123456789Z[Custom/TimeZone]")
+                .epochNanoseconds
+        ).toBe(160583136123456789n);
+
+        // Accepts but ignores the calendar.
+        let result = null;
+        expect(() => {
+            result = Temporal.Instant.from("1970-01-01T00:00Z[u-ca=UTC]");
+        }).not.toThrow();
+        expect(result).toBeInstanceOf(Temporal.Instant);
+        expect(result.epochNanoseconds).toBe(0n);
+
+        // Does not validate calendar name, it only checks that the calendar name matches the grammar.
+        result = null;
+        expect(() => {
+            result = Temporal.Instant.from("1970-01-01T00:00Z[u-ca=aAaAaAaA-bBbBbBb]");
+        }).not.toThrow();
+        expect(result).toBeInstanceOf(Temporal.Instant);
+        expect(result.epochNanoseconds).toBe(0n);
+    });
+});
+
+describe("errors", () => {
+    test("invalid instant string", () => {
+        expect(() => {
+            Temporal.Instant.from("foo");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+
+    test("invalid epoch nanoseconds", () => {
+        // Test cases from https://github.com/tc39/proposal-temporal/commit/baead4d85bc3e9ecab1e9824c3d3fe4fdd77fc3a
+        expect(() => {
+            Temporal.Instant.from("-271821-04-20T00:00:00+00:01");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date");
+        expect(() => {
+            Temporal.Instant.from("+275760-09-13T00:00:00-00:01");
+        }).toThrowWithMessage(
+            RangeError,
+            "Invalid epoch nanoseconds value, must be in range -86400 * 10^17 to 86400 * 10^17"
+        );
+    });
+
+    test("annotations must match annotation grammar even though they're ignored", () => {
+        expect(() => {
+            Temporal.Instant.from("1970-01-01T00:00Z[SerenityOS=cool]");
+        }).toThrowWithMessage(RangeError, "Invalid ISO date time");
+    });
+});