Add XO linter

[LitoMore]

I intentionally didn't fix those errors on the commit 64fefbd (#10643), so you can play XO.

Changes

• Apply most of the XO fixes. For some minor errors, I used eslint-ignore.
• Move the .svglintrc.mjs code body to scripts/svglint-config.mjs. It's related to XO not linting files starts with dot xojs/xo#673.
• Remove prettier dependency, XO has a built-in prettier
• Limit the prettier --fix command to only format markdown and JSON files
• Remove the incorrect engines field from package.json

Features

It has a friendly error prompt, and you can simply the link on the prompt to see the reason:

It has tons of rules and good practices built in.

Let me know if you have any questions.

Editor configuration

VS Code .vscode/settings.json

Some options can be omitted. It's up to your global configuration.

{
  "prettier.enable": true,
  "eslint.enable": false,
  "xo.enable": true,
  "xo.format.enable": true,
  "editor.formatOnSave": true,
  "[javascript]": {
    "editor.defaultFormatter": "samverschueren.linter-xo"
  },
  "[json]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  },
  "[markdown]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  }
}

For other editors, please refer to https://github.com/xojs/xo?tab=readme-ov-file#editor-plugins.

[mondeja]

Really great, you're right about XO 🥇

[mondeja]

It's almost there, just one more small change...

[mondeja]

Awesome. Thanks for this great work @LitoMore! 🥇