chore: add gen-linkml recipe

[noelmcloughlin]

This PR adds gen-linkml recipe to project.justfile to address #167

Notes:

• The recipe is not invoked automatically yet (maybe should be added to gen-project or promoted later).
• Use Group "model development - extended" to support Track maturity of Linkml generators #166 candidates

Closes #167

[dalito]

Works as it should. Nice addition as it demonstrates how to extend the pool of just recipes,