Add XO linter

[LitoMore]

Changes

• Remove .vscode/settings, and ignore it from .gitignore
• Add XO linter

Should we ignore the .vscode/settings.json file?

Of course, we should.

We should give developers more free space to customize their editors.

BTW, all Sindre's open-source projects do the same behavior.

How to activate this linter in VS Code

1. Install XO extension to your VS Code

2. Create a .vscode/settings.json file

3. Paste the content below to .vscode/settings.json

   {
   	"xo.enable": true,
   	"xo.format.enable": true,
   	"editor.formatOnSave": true,
   	"editor.defaultFormatter": "samverschueren.linter-xo"
   }
   

4. Reload or restart your VS Code