fix: support context of webpack.config

[daichi-yamauchi]

Fix that specifying the context of webpack.config cause no such file error.

Occurred error

• webpack.config.js

module.exports = {
  //...
  context: path.resolve(__dirname, 'app'),
};

Run npx webpack

Error: ENOENT: no such file or directory, stat 'fonts/my-font.ttf'