Add a CI job that checks if the list of historical stdlibs needs to be updated

[DilumAluthge]

This pull request add a new CI job that does the following steps:

1. Checks that the working directory is clean.
2. Instantiates the project at ext/HistoricaStdlibGenerator.
3. Runs the ext/HistoricaStdlibGenerator/generate_historical_stdlibs.jl script.
4. Checks that the working directory is still clean. If the working directory is dirty, we do the following:
   • Print the output of git status.
   • Print the output of git diff HEAD.
   • Throw an exception, thus causing the CI job to fail.

[DilumAluthge]

Note that the check is (correctly) failing.

[IanButterworth]

It would've been good to have rebased this before merging.
Looking at the failure on the merge there appears to be a problem with generate_historical_stdlibs.jl that may have been introduced with #2795

[IanButterworth]

The problem with the 1.8.0 entry is because JuliaLang/julia#42806 hasn't landed yet. It's splitting the stdlib name string into first and second character, instead of the two parts of a (name, version) tuple.

[DilumAluthge]

Alright, I've merged JuliaLang/julia#42806.

Can I re-run the generate_historical_stdlibs.jl script now? Or do I need to wait until the nightlies are updated?

[IanButterworth]

Ah of course, the action can just be re-run.
Seems to run off nightlies so will need to wait

[DilumAluthge]

Alright, I'll wait for the nightlies to update, and then I'll re-run the generate_historical_stdlibs.jl script and make a PR.