extensions: add 'basic-graphics' extension

[mboudr35]

This extension provides a shortcut for snaps that make use of the 'mesa-core22' snap of graphics libraries & resources. It configures the conventional 'graphics-core22' content plug and provides the 'graphics-launch' helper script for setting-up the environment.

Those that make frequent use of the 'mesa-core22' snap when building snaps intended for execution on Ubuntu Core 22 (or eventually later) running ubuntu-frame may especially appreciate this extension, as this automates much of the boilerplate put in the snapcraft.yaml.

• [x] Have you followed the guidelines for contributing?
• [x] Have you signed the CLA?
• [x] Have you successfully run make lint?
• [x] Have you successfully run pytest tests/unit?

---