Modify module import and export

[HidemichiShimura]

Description

• Modify module import and export in server.js and chatPostMessage.js
• Modification from #3

Implementation

• Replace require() with import
• Replace module.exports with export

Note

I'm going to modify module loading const lookupRandomCocktailName = require("./functions/lookupRandomCocktail") in another pull request.

[HidemichiShimura]

Thank you for your review!