gh-90716: Refactor PyLong_FromString to separate concerns (GH-96808)

This is a preliminary PR to refactor `PyLong_FromString` which is currently quite messy and has spaghetti like code that mixes up different concerns as well as duplicating logic.

In particular:

- `PyLong_FromString` now only handles sign, base and prefix detection and calls a new function `long_from_string_base` to parse the main body of the string.
- The `long_from_string_base` function handles all string validation and then calls `long_from_binary_base` or a new function `long_from_non_binary_base` to construct the actual `PyLong`.
- The existing `long_from_binary_base` function is simplified by factoring duplicated logic to `long_from_string_base`.
- The new function `long_from_non_binary_base` factors out much of the code from `PyLong_FromString` including in particular the quadratic algorithm reffered to in gh-95778 so that this can be seen separately from unrelated concerns such as string validation.