Type hints: basic usage to infer argument types (fixes #203)

[neithere]

MVP:

• [x] add support for str, int, float (and their deprecated aliases: Str, etc. until we drop support for Python 3.8 around Q4 2024)
• [x] docs with examples (tutorial / cookbook)

Nice to have:

• nargs=ZERO_OR_MORE:
  • [x] list[str] → nargs=ZERO_OR_MORE, type=str
  • (same for list[str] | None, list[str | None] and list[str | None] | None — there's no difference for us in the context of mapping).
• nargs=N
  • [ ] foo: Tuple[str, int, float] -> add_argument('foo', nargs=3) followed by (e.g.) args = parser.parse_args(); args.foo[1] = int(args.foo[1]); args.foo[2] = float(args.foo[2]) (https://github.com/neithere/argh/issues/107#issuecomment-843334649)
• Choices:
  • [x] foo: Literal['one', 'two', 'three'] -> add_argument('foo', choices=['one', 'two', 'three'] possibly with completion (later). (https://github.com/neithere/argh/issues/107#issuecomment-843334649)
• Passing add_argument() params via hints instead of @arg:
  • [ ] Annotated[x, ExtraParams(type=some_callable, help="something", nargs="+")]