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

vlib/strconv/atof.js.v

@@ -0,0 +1,9 @@
+module strconv
+
+// atof64 return a f64 from a string doing a parsing operation
+pub fn atof64(s string) f64 {
+	res := 0.0
+	#res.val = Number(s.str)
+
+	return res
+}