Implement support for parsing custom at rules in JS bindings

[devongovett]

Closes #391

This adds a new configuration option to the Lightning CSS JavaScript API which enables basic parsing of custom at-rules into a more structured AST rather than storing raw tokens. You can configure how the prelude and block should be parsed, and then use the value to transform the rule if needed. The syntax for the prelude is defined using a syntax string (as used in the @property rule), and the block can be a "declaration-list", "rule-list", or "style-block" as defined in the css spec.

In this example, @breakpoints accepts a rule list as a body, and @test accepts a style block, i.e. either a declaration list (in nested style rules) or a rule list. @test also expects a comma separated list of identifiers as a prelude.

let res = css.transform({
  code: Buffer.from(`
  @breakpoints {
    .foo { color: yellow; }
  }

  .foo {
    color: red;
    @test foo, bar {
      width: 25px;
    }
  }
`),
  customAtRules: {
    breakpoints: {
      body: 'rule-list'
    },
    test: {
      prelude: '<custom-ident>#',
      body: 'style-block'
    }
  },
  // ...

To do

• Docs
• Tests