Implement Neural Actions (prompt snippets)

As we use GPT models, we may come across good prompts that derive the outputs we want e.g.
* `Write a PlantUML diagram for X`
* `Summarise X`
* `Act as an expert in X and explain Y`
* Create a commit message for this diff
etc

It would be helpful for people to save these and bring them forward with a custom dropdown behaviour or via a "completion source". Particularly useful for prompt building for #19