Based on the Haskell lexer but adapted to match PureScript's actual
syntax as defined in the compiler's CST lexer. Key differences: added
keywords (ado, derive, forall, foreign), typed holes (?name), triple-
quoted raw strings, Unicode operator equivalents (∀→←⇒∷), no binary
or octal literals, no Template Haskell, simplified escape sequences,
and a different import syntax where the import list precedes "as".