Allow variant configuration overrides

[Krabba]

Previously, the theme variants inherited the tree and types from the main theme, so you couldn't re-assign theme variants if the type is literal.

Changelog

• With this feature branch, it allows overrides on the theme from a variant perspective:

const theme = new Theme({
  colors: {
    somecolor: "#e9e6ff" as const,
  },
});

const darkMode = theme.variant({
  colors: {
    somecolor: "#000000", // This is now valid in TypeScript
  },
});

[ankarhem]

I think we could use the LiteralToPrimitiveDeep from type-fest instead.