Add '$llm' RHS label expression

[ihh]

Expands arguments (allowing insertion of text from subgraph labels) and then feeds to LLM as a prompt.

[ihh]

Partially implemented using execSync to call llm, as of 52aac7a.

A better solution would be to call the LLM web API directly; this would require the Grammar.evolve method to be async (see #7).