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Typed Python §4, phase 1: the parser accepts type annotations (#633)

* Typed Python §4, phase 1: the parser accepts type annotations

Python §4 with type annotations (docs/typed-python-4.md, PR #630). The parser now accepts:

* annotations of the parameters of a def (`x: int`, `*xs: int`, `x: int = 1`), a return
  annotation (`-> T`; `->` is now one token) and type parameters (`def f[A, B](...)`),
* `type Name[A] = T` statements (`type` stays a soft keyword: `type = 1` is an assignment),
* a subscript with several items (`Callable[[int], int]`, `Pair[int, int]`), as in Python.

AST: FunctionParam.annotation, FunctionDef.returns and typeParams, a new TypeAlias statement.
The parameters of a lambda cannot be annotated, as in Python.

Validators: Python §1-§4 reject the new syntax (NoTypeAnnotationsValidator, with the existing
NoAnnAssignValidator), so nothing changes for them. makeTypedChapter4Validators() holds the rules
of Typed §4: every def annotates its parameters and its return type, and a declaration needs a
value. prepareTypedProgram (src/typecheck/) checks them, and eraseTypes then strips the types
(annotated assignments become assignments, type statements become pass), so that every engine
runs the program as a Python §4 program.

Tests: parsing, rejection in §1-§4, the typed rules, erasure, and the same output with and without
types on the CSE machine, py2js and PVML in the browser.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* full e-stepper: do not run annotated functions and type statements

The parser now accepts the annotations of a def, type parameters and `type` statements (Typed
Python §4). CPython evaluates annotations when the def runs, which this engine does not do yet, so
`def f(x: Undefined)` would print the result instead of raising a NameError. Report them as
unsupported, as annotated assignments already are (and only when the statement is reached).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

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