Write the JavaScript styleguide

For the moment our JavaScript styleguide exists implicitly and can only be inferred from reading our source code. The eslint config that we use in our projects covers the most essential parts of the code style, but a great deal of style-related things cannot be checked by linter, though the eslint config should definitely be placed in this repo too. We need to document our JavaScript code style and save these conventions both there and in Metarhia repositories.

首先檢視現有的 JavaScript 原始碼，以及專案中使用的 eslint 設定，以找出未經 lint 檢查的慣例。將這些慣例記錄在 styleguide 中，將 eslint 設定加入此 repository，並確保產生的指引可以儲存到 Metarhia repositories 中。