lang/janet: Update to 1.31.0.

## 1.31.0 - 2023-09-17

- Report line and column when using `janet_dobytes`
- Add `:unless` loop modifier
- Allow calling `reverse` on generators.
- Improve performance of a number of core functions including `partition`, `mean`, `keys`, `values`, `pairs`, `interleave`.
- Add `lengthable?`
- Add `os/sigaction`
- Change `every?` and `any?` to behave like the functional versions of the `and` and `or` macros.
- Fix bug with garbage collecting threaded abstract types.
- Add `:signal` to the `sandbox` function to allow intercepting signals.

(charlotte)