Implement Neural Actions (prompt snippets)

[Angelchev]

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