bump Lean version to 4.29.0, the current stable version

[oxe-i]

Bumps Lean version from 4.25.2 to 4.29.0, which is the current stable version.

In order to make this process as sane as possible, I've made the following changes:

1. Changed the generator script so that -r (regenerate) now copies the toolchain in templates/lean-toolchain to each exercise. This ensures they are synchronized and also makes it much easier to bump the toolchain version for all exercises, by just changing the template toolchain.
2. While at it, I also improved the messages to indicate the exercise being currently generated, in order to make it possible to act on it in case of an error.
3. Bumped the version for the generator script too, so that we don't need to maintain two different toolchains.
4. Added --wfail to both bin/run-generator and bin/verify-exercises, so that any warning is treated as an error.
5. Changed the generator script and all other files (test generators and example solutions) that reported an error. I think most of them were due to changes in the String API functions. For example, List.asString being deprecated in favor of String.ofList. However, there were two example solutions that needed a deeper rework: word-count and wordy.
6. Found and fixed a bug in the generator script for nucleotide-count. I was extracting the expected counts even in case of a test case where the result was an error and not a valid count.

I'll bump the version in the test-runner next.

[keiravillekode]

Approved to merge around the same time the test runner PR is merged.

[keiravillekode]

We should post an update in https://forum.exercism.org/t/string-split-and-acronym/36697