feat(theme): generate layer if option is provided

[Niki2k1]

Description

I was trying to get Tailwind working with Vuetify and enabled layers. I had some issues with the theme overwriting my tailwind colors and such. So I created an option which lets me enable a layer for the generated theme.

Usage:

theme: {
  unimportant: true,
  layer: true, // or specify a custom name: 'my-custom-layer'
},

which then generates:

@layer vuetify-theme { 
 :root {
      --v-theme-background: 255,255,255;
      --v-theme-background-overlay-multiplier: 1;
      --v-theme-surface: 255,255,255;
      --v-theme-surface-overlay-multiplier: 1;
      /* ... */