Implement CSS Cascade Layers support

[AlexandrHoroshih]

Closes theKashey#50

What is implemented

• @layer rules got special processing, similiar to @media rules
• AST types are refactored to make a clear distinction between processed at-rules (like @media or @layer, which are handled in a custom way) and unprocessed at-rules (any other "unknown" rule, which will be dropped into critical-css as-is)

What is not implemented

There is a possible edge-case:
It is possible to have the same layers order definition at multiple files, something like this

/* index.css */
@layer a, b, c;

// ...styles

/* chunk.css */
@layer a, b, c;

// ...styles

I think, it is possible in the situation, where layers are added automatically by bundlers, based on some heuristics or rules, since this way layers order is guaranteed and is not dependent on loading order 🤔

In that case used-styles probably should deduplicate these definitions.

But since i do not have any actual real-life use-cases, which would require something like this, it is not added in this PR

If this problem will actually happend one day - then cross-file deduplication of layer definitions can be solved by introducing something like "shared context" for the AST parser:

const astFromFiles = (fileDate: StyleFiles): StyleAst => {
  const sharedContext = {}

  return Object.keys(fileDate).reduce((acc, file) => {
    acc[file] = buildAst(fileDate[file], file, sharedContext);

    return acc;
  }, {} as StyleAst);
};

const buildAst = (cssString, file, context = {}) => {
  // ...
 if (context.layerDefinitions.has(`@layer ${rule.params}`) {
  return;
 }
}