Function _isLetter

lib/css/syntax.js:319–321

(cc)

 * @returns {boolean} is letter (a-z / A-Z)
 */
const _isLetter = (cc) =>
	(cc >= CC_LOWER_A && cc <= CC_LOWER_Z) ||
	(cc >= CC_UPPER_A && cc <= CC_UPPER_Z);

/**
 * Spec: ident-start = letter / non-ASCII / `_`. Internal helper that