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.

首先检查现有的 JavaScript 源代码以及项目中使用的 eslint 配置，以识别未通过 lint 检查的约定。将这些约定记录在 styleguide 中，将 eslint 配置添加到此 repository，并确保由此形成的指导可以保存到 Metarhia repositories 中。