feat(npm): Add TypeScript types to @fastly/cli

[harmony7]

This PR adds TypeScript types to the @fastly/cli package so that it can be imported into TypeScript programs.

import cli from '@fastly/cli';

const result = spawnSync(
  cli,
  [
    "version",
  ],
  {
    shell: true,
    encoding: "utf-8",
  }
);

Without these types, TypeScript will complain:

TS2306: File
/path/to/node_modules/@fastly/cli/index.d.ts
is not a module.