Fail CI if rake swig causes changes

[jacobvosmaer]

Closes #136

This adds a CI check that will cause a failed check if we accidentally commit a _wrap.cxx file that does not match the corresponding .i file.

[jacobvosmaer]

Here is an example failure, triggered by an intentionally wrong modified wrap file:

https://github.com/robinst/taglib-ruby/actions/runs/8602424269/job/23571960305?pr=139

I will remove the "wrong" test commit now.

[robinst]

Nice, thanks! Merged.