refactor(api): Merge `api` into `edge` package

[koistya]

• Merge api package into edge, see edge/routes/api.ts
• Emulate Cloudflare Workers environment in unit tests using Miniflare
• Add SendGrid client for sending emails from under Cloudflare Worker script
• Add SENDGRID_API_KEY, FROM_EMAIL environment variables

NOTE: Cloudflare Worker script(s) need to remain lightweight. If you need to implement a full-featured API, take a look into kriasoft/relay-starter-kit → /api as an example (using Node.js, PostgreSQL, and GraphQL.js).