feat: add core/plugin entrypoints

[tmm]

Adds emulate/core and emulate/plugins entrypoint re-exports so downstream projects can compose custom emulators or extend built-in ones without needing to install and manage @emulators/* packages directly.

Extend a built-in emulator with a missing GitHub Contents API endpoint:

import { createPlugin, createServer, serve } from 'emulate/core'
import { githubPlugin } from 'emulate/plugins'

const plugin = createPlugin({ // `createPlugin` API added for simpler typing
  name: 'github',
  register(app, store, webhooks, baseUrl, tokenMap) {
    // Register the built-in GitHub emulator first.
    githubPlugin.register(app, store, webhooks, baseUrl, tokenMap)

    // Add a downstream-only endpoint that your app or SDK needs.
    app.get('/repos/:owner/:repo/contents/:path{.+}', (c) => {
      const owner = c.req.param('owner')
      const repo = c.req.param('repo')
      const path = c.req.param('path')
      const content = `# ${repo}\n\nLocal fixture for ${owner}/${repo}.\n`

      return c.json({
        type: 'file',
        name: path.split('/').at(-1),
        path,
        sha: 'local-fixture',
        encoding: 'base64',
        content: Buffer.from(content).toString('base64'),
        download_url: `${baseUrl}/repos/${owner}/${repo}/raw/${path}`,
      })
    })
  },
  seed: githubPlugin.seed,
})

const { app } = createServer(plugin, { baseUrl: 'http://localhost:4000' })
const server = serve({ fetch: app.fetch, port: 4000 })

Create a fully custom emulator:

import { createPlugin, createServer, serve } from 'emulate/core'

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

const { app } = createServer(plugin, { baseUrl: 'http://localhost:4100' })
const server = serve({ fetch: app.fetch, port: 4100 })

[vercel]

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