new based syntax

stringizer will kill us, but anyway:

cpython compatible based syntax:
- `(str) > int` for callable syntax, annoyingly `ast` wont see the parens here
- `"foo" | "bar"` for string literals, will need to be a per module setting (also need a fallback option for forward refs #361)
- `f"a_{int}"` for template literals or w/e we call them

custom only based syntax:
- `def f[T: Overload[str, int]]():` for constrained types `T: (str, int)` is stupid
- `(int=, /, *, foo: str=, **bar: str) -> int` fullform callable syntax (`=` or `=...` for default?)

The issue is a proposal for several new based syntax forms, including callable syntax, string literal alternatives, template literals, constrained types, and full-form callable signatures. No files, tests, or entry points are identified; first clarify the intended subset and per-module or fallback behavior. Done would require an agreed design and implementation scope for the selected syntax.

Written by the indexing model from the issue text.