PP-12343: Add post merge package and release, and ensure package works on PRs

[jfharden]

1. Add a .pkl-version to the root of the repo which is loaded by all GitHub actions to decide the version of pkl to install
2. In the unit tests use that version file, and also ensure the pkl project package can successfully run in the module
3. Add a build directory which is gitignored
4. Add a note to the README about how to release new versions
5. Fix the baseUri in the PklProject
6. Add post-merge workflow which will:
   1. Check if the version in the PklProject has a release already
   2. If there is not already a release run the pkl project package command; and
   3. Create a github release with the package artifacts

In order to test the post-merge workflow I set it to run on the PR, but to print debug output instead of creating the release, you can see a full run of it like that: https://github.com/alphagov/pkl-concourse-pipeline/actions/runs/8142949291/job/22253656509

I'll do another follow-on PR to update the README once I can write working tests using the published version.
I'll also squash the multiple commits in this PR when I merge it to save from the pointlessly noisy commit history

[kerrr]

LGTM 🚀