Support for the `@layer` rule

To reproduce:

@layer reset, preflights, layout, atom // This doesn't work: compile error at `,`

// These work fine because of how stylus handles unknown @-rules
@layer reset
	@import 'reset'

@layer preflights
	@import 'vars'
	@import 'common'

@layer layout
	@import 'layout'

@layer atom
	@import 'atom'

Current behavior:

Cannot compile @layer reset, preflights, layout, atom because of an unexpected character.

Can be worked around by using a css literal:

@css {
	@layer reset, preflights, layout, atom;
}

Expected behavior:

It should be able to compile like in this example.

Environment information:

• stylus version: 0.61.0
• nodejs version: v18.18.2

Sorry it's a super nitpiky issue because the workaround is super simple. I just thought it should be supported thanks to how much of a lifesaver css layers are by ignoring all specificity.