add normal HTTP server listener for debugging purposes

[syumai]

What

• This PR adds normal HTTP server listener for debugging purposes.
• With this feature, the example below works using only the go run command without GOOS and GOARCH.

package main

import (
	"fmt"
	"net/http"

	"github.com/syumai/workers"
)

func main() {
	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
		fmt.Fprint(w, "Hello, world!")
	})
	workers.Serve(handler)
}

Motivation

• To make development easier for developers who are not familiar with JS/Wasm.