[pigment-core] Create new UI framework independent package

[brijeshb42]

@pigment-css/core

1. Supports css() and keyframes() api
2. Add preprocessor to generate keyframes' correct output
3. processors/css.ts encompasses all the core logic that'll be inherited by styled in @pigment-css/react as well.
4. All generated css are part of different layers pigment.base, pigment.variants and pigment.compoundvariants with preference in the same order.
5. In each css() call, users can define how the class name is to be generated instead of this being a top-level pigment config, ie,

// cls's value will be random but deterministic across builds
const cls = css`
  color: red;
`;

// cls's value will always be "logo"
const cls = css({className: 'logo'})`
  color: red;
`;

TODOS

• globalCss implementation

• I have followed (at least) the PR section of the contributing guide.