feat(icons): re-include old css3 icon (unassociated)

[sgoudham]

This PR re-introduces the css3/css shield icon to give users the option of using the previous default icon.

To revert back to the previous default icon, add the following JSON to your settings.json:

"catppuccin-icons.associations.extensions": {
  "css": "css3"
},
"catppuccin-icons.associations.languages": {
  "css": "css3"
},

Closes #444