[Feature] add file path extname resolve

[alanchenchen]

butterpack will could resolve file path extname such as:

import { equal } from "utils"; // ext name is .ts
import icon from "icon"; // ext name is .png
import Count form "./Count" // ext name is .tsx

// butterpack.config.js
export default {
 fileExts: [".ts", ".tsx", ".png"]
}

default ext name is .js