Latest `grafast` depends on `graphile-config` but doesn't list dependency
```
yarn add grafast@beta
```

Run this:
```ts
// @ts-check
import { makeGrafastSchema, lambda, grafastSync } from "grafast";

const schema = makeGrafastSchema({
  typeDefs: `
  type Query {
  tweaker(hoofer: [String!]!): Int
  }
  `,
  plans: {
    Query: {
      tweaker(_, { $hoofer }) {
        return lambda($hoofer, (hoofer) => hoofer.length);
      },
    },
  },
});

const result = grafastSync({
  schema,
  source: /* GraphQL */ `
    query MyQuery($hoofer: [String!]!) {
      tweaker(hoofer: $hoofer)
    }
  `,
  variableValues: {
    hoofer: ["a", "b"],
  },
});

console.dir(result);
```

Get this:

```
Error: Cannot find module 'graphile-config'
Require stack:
- /home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/middleware.js
- /home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/execute.js
- /home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/index.js
    at Module._resolveFilename (node:internal/modules/cjs/loader:1144:15)
    at Module._load (node:internal/modules/cjs/loader:985:27)
    at Module.require (node:internal/modules/cjs/loader:1235:19)
    at require (node:internal/modules/helpers:176:18)
    at Object.<anonymous> (/home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/middleware.js:4:27)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1435:10)
    at Module.load (node:internal/modules/cjs/loader:1207:32)
    at Module._load (node:internal/modules/cjs/loader:1023:12)
    at Module.require (node:internal/modules/cjs/loader:1235:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/middleware.js',
    '/home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/execute.js',
    '/home/benjie/Dev/DELETEME/sje-null/node_modules/grafast/dist/index.js'
  ]
}
```

Probably it should only use the graphile-config if it exists - should be optional dependency?