chore: add .editorconfig

[ilharp]

https://editorconfig.org/

[MaikoTan]

I think this would be more like a "personalized setting" not a general way to force contributors to follow the coding style.
Since we have ESLint for that already, I don't see the necessity of adding this file.

[ilharp]

All settings in .editorconfig are not "personalized setting"; they're basic rules that Koishi use, and should to force contributors to follow - insert_final_newline, indent_style, indent_size etc.

https://editorconfig.org has a full description of the benefits of EditorConfig. EditorConfig is used more for the editor to automatically recognize coding styles, not for code linting.

ESLint can do linting but it won't modify editor preferences, which creates a big divide between code editing and yarn lint. Editor's default settings can sometimes lead to a bad experience. EditorConfig is for solving this.