NilPy: ClassVar storage, next(), counter shim, lambda default_factory

Part of [[feature-nilpy-corpus-uforth]] milestone 1 — the current uforth parse wall (Word.xt_id):

xt_id: int = field(default_factory=lambda: next(Word._xt_counter))
_xt_counter: ClassVar = None    # later: Word._xt_counter = _count(1)

Chain needed: (1) ClassVar class-level statics readable/writable as Class.name (pxx has Pascal class var machinery — FindClassVar — to map onto); (2) an itertools.count shim (pylib TPyCounter with a next method); (3) next(x) builtin -> x's next method; (4) field(default_factory=<expr>) evaluating <expr> per construction — the general form beyond the shipped =list case; a restricted zero-arg lambda body is enough for uforth.

Semantics to preserve: unique increasing xt ids per Word creation.