Add support for extensionAlias

[PSoltes]

Add support to add extensionAlias config in serverless yml file. It allows for using nodenext module resolution with webpack. Should be quite straight forward fix (just passing array of objects to webpack config from serverless yml)
https://webpack.js.org/configuration/resolve/#resolveextensionalias

extensionAlias:
  js:
    - js
    - ts
  jsx:
    - jsx
    - tsx

and then just plop this straight up to src/webpack.config.js:441 if it exists