Add editorconfig, prettier and sync lint rules with GNOME Shells

[Leleat]

I am not sure how I feel about adding prettier yet. I like the general formatting of prettier but a) Tiling Assistant hasn't used it before and b) I've never seen prettier used in a GNOME context before... I'll need to mull it over...

[jtojnar]

Apparently, ESLint is moving away from stylistic rules so prettier will probably be necessary: https://typescript-eslint.io/troubleshooting/formatting/#eslint-core-and-formatting