Add demo of indirection syntax to support ‘variable variables’

[maetl]

This adds new modifier syntax supporting a basic metaprogramming feature where the result of a template expansion is itself used as a template expansion term rather than being concatenated into the result.

eg:

const varvar = calyx.grammar({
  start: "{~name} from the {~location}",
  "location": "{setting}",
  "name": "{setting}_names",
  setting: ["fantasy", "scifi", "cyberpunk"],
  fantasy: ["tower", "dungeon", "village"],
  scifi: ["starship", "mining colony", "ringworld"],
  cyberpunk: ["neon city", "pharma lab"],
  fantasy_names: ["Undin", "Mirielye", "Edhen", "Burga"],
  scifi_names: ["Raige", "Enjan", "U'Krada", "Chrotha"],
  cyberpunk_names: ["Barick", "Zechiel", "Golden Static", "Citizen Death"]
});

const result = varvar.generate();