new(CI): Add informational API diff job (packdiff) for the library packages

[zlonast]

Adds an informational "Check API" job that shows, on every pull request, how the
public API of Cabal-syntax, Cabal, cabal-install-solver and Cabal-hooks
changes with respect to the PR base.

The job is not a required check and does not block merging; it exists to:

• make API changes visible during review,
• inform changelog entries and PVP version bumps,
• inform backport decisions for release branches (a red job means "probably not
  a backport candidate").

$ make api-install                               # once; installs packdiff from the pinned commit
$ make api-diff                                  # all four packages, against origin/master
$ make api-diff PKG=Cabal-syntax API_BASE=3.14   # one package, against a branch/tag

---

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• The Check API job is clean, or the API changes are documented in a changelog file.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[zlonast]

@ulysses4ever Hi, what do you think of this version of the check API?

[ulysses4ever]

This looks good. Im happy to approve after you strike the test changes (apiCheck). Maybe take a screenshot for other reviewers . Two things I can suggest:

• I don’t believe you need a whole new project file to install a package from GitHub: just fetch the tar, untar, cd into the directory and do it!
• I’d add a checkbox into the PR template regarding this workflow. E. g. “Either the api-check is clean or the api changes are documented in the changelog file”