Added a CI workflow that runs the examples

[Helveg]

Hi, I've added the basic structure to run unit tests during CI.

The only test that I've added tests that all the files in the example folder are runnable without errors, for several supported matplotlib and seaborn versions.

The CI run also revealed that starbars has the following minimum version requirements:

• Python 3.9+
• matplotlib 3.9.0+
• seaborn 0.13.0+

[elide-b]

thank you :)