LibJS: Throw RangeError in StringPrototype::repeat if OOM

currently crashes with an assertion failure in `String::repeated` if
malloc can't serve a `count * input_size` sized request, so add
`String::repeated_with_error` to propagate the error.

AK/String.h

@@ -79,7 +79,7 @@ public:
     static ErrorOr<String> repeated(u32 code_point, size_t count);
 
     // Creates a new String from another string, repeated N times.
-    static String repeated(String const&, size_t count);
+    static ErrorOr<String> repeated(String const&, size_t count);
 
     // Creates a new String by case-transforming this String. Using these methods require linking LibUnicode into your application.
     ErrorOr<String> to_lowercase(Optional<StringView> const& locale = {}) const;