strconv,v.gen.js: proper optionals returns, strconv compiles to the JS backend (#11364)

vlib/builtin/js/utf8.js.v

@@ -0,0 +1,9 @@
+module builtin
+
+pub fn utf8_str_visible_length(s string) int {
+	// todo: proper implementation
+	res := 0
+	#res.val = s.str.length;
+
+	return res
+}