feat: emulate.config.ts

[tmm]

Adds defineConfig and emulate.config.ts / .js loading so the CLI can launch custom or extended emulator plugins. Existing YAML and JSON seed configs still work through --seed and autodiscovery.

Use emulate.config.ts to extend a built-in emulator from the CLI, including custom ports and seed data. This example adds GitHub's Contents API for a downstream app while preserving the built-in GitHub routes.

import { defineConfig } from 'emulate'
import { createPlugin } from 'emulate/core'
import { githubPlugin } from 'emulate/plugins'

const github = createPlugin({
  name: 'github',
  register(app, store, webhooks, baseUrl, tokenMap) {
    // Preserve built-in GitHub behavior.
    githubPlugin.register(app, store, webhooks, baseUrl, tokenMap)

    // Add GitHub's Contents API, which this app uses but emulate does not support yet.
    app.get('/repos/:owner/:repo/contents/:path{.+}', (c) => {
      const path = c.req.param('path')
      return c.json({
        name: path.split('/').at(-1),
        path,
        type: 'file',
        encoding: 'base64',
        content: 'SGVsbG8gZnJvbSBlbXVsYXRlCg==',
      })
    })
  },
  seed: githubPlugin.seed,
})

export default defineConfig({
  services: {
    github: {
      plugin: github,
      // Override the default auto-incremented port for this service.
      port: 4001,
      seed: { users: [{ login: 'octocat' }] },
    },
  },
})

The same config file can register a fully custom emulator service that is not part of the built-in registry. This keeps app-specific test APIs available through npx emulate without publishing a separate emulator package.

import { defineConfig } from 'emulate'
import { createPlugin } from 'emulate/core'

const custom = createPlugin({
  name: 'custom',
  register(app) {
    // Define whatever API surface your app needs for tests.
    app.get('/ping', (c) => c.json({ ok: true }))
  },
})

export default defineConfig({
  services: {
    custom: {
      plugin: custom,
      port: 4100,
    },
  },
})

Needs vercel-labs#178