Create separate exports for walker and nodes

[bartveneman]

use case: I want to do this, but I have to import the entire module instead

import { parse_atrule_prelude } from '@projectwallace/css-parser/parse-atrule-prelude'
import { walk, SKIP } from '@projectwallace/css-parser/walker' // <- fails
import { LAYER_NAME } from '@projectwallace/css-parser/nodes' // <- fails

const ast = parse_atrule_prelude('import', params)

for (const node of ast) {
	if (node.type === LAYER_NAME && !node.name) {
		utils.report({
			result,
			ruleName: rule_name,
			message: messages.rejected(),
			node: atRule,
			word: 'layer',
		})
	}
}

[bartveneman]

implemented in #196