Adds sb.jinja helper

[liamgriffiths]

This PR implements the sb.jinja helper. It allows a user to produce FutureString (soon Future<string> #93) values with the result of rendering a Jinja2 template.

For example,

import { Substrate, Box, sb } from "substrate";

async function main() {
  const SUBSTRATE_API_KEY = process.env["SUBSTRATE_API_KEY"];

  const substrate = new Substrate({
    apiKey: SUBSTRATE_API_KEY
  });

  const a = new Box({ value: ["a", "b"] });
  const b = new Box({ value: { x: "x" } });

  const f = sb.jinja('as=[{% for a in as %}{{a}},{% endfor%}], b={{b["x"]}}, c={{c}}', {
    as: a.future.value,
    b: b.future.value,
    c: "1234",
  });

  const c = new Box({ value: f });

  const res = await substrate.run(a, b, c);
  console.log(JSON.stringify(res.json, null, 2));
}
main();