Unify linter and formatter definitions

The most promising thing seems to be biome. The teamlead is employed by astro. It's a very young project though.

Its goal is to unify linting and formatting for all common languages, and to allow interactive linting/formatting thanks to its very high speed and efficiency.

When we use biome, we can get rid of prettier and eslint (and eventually stylelint), and have less dependencies and configs.