LibJS: Support large number of decimals in Number.prototype.toFixed

The spec asks us to perform some calculations that quickly exceed an
`u64`, but instead of jumping through hoops we can rely on our AK
implementation of floating point formatting to come up with the
correctly rounded result.

Note that most other JS engines seem to diverge from the spec as well
and fall back to a generic dtoa path.

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toFixed.js

@@ -31,6 +31,21 @@ describe("correct behavior", () => {
     });
 });
 
+describe("large number of digits", () => {
+    test("maximum", () => {
+        expect((1).toFixed(100)).toBe(
+            "1.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+        );
+        expect((-3).toFixed(100)).toBe(
+            "-3.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
+        );
+    });
+
+    test("fractional values", () => {
+        expect((1.5).toFixed(30)).toBe("1.500000000000000000000000000000");
+    });
+});
+
 describe("errors", () => {
     test("must be called with numeric |this|", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {