LibJS: Don't validate time zone name when parsing Instant string

This is normative change in the Temporal spec.

See: 2a81fbc

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

@@ -18,6 +18,11 @@ describe("correct behavior", () => {
         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);
     });
 });