[WIP] support async imports in npm modules

[collin]

Some dependencies make use of async imports: import('module-name').then((defaultExport) => {})

Students who run into this issue are hit particularly hard, as it's not obvious why installing a module would cause this bug. Also it involves path construction issues which are particularly difficult to master.

This results in chunked bundles 0.bundle.js.

As configured, webpack will assume these should be fetched as GET /public/0.bundle.js

But the URL should be GET /0.bundle.js.

• [x] support serving split bundles in development mode
• [x] support deploying split bundles in production mode (will require update to deploy script to find these file)
• [x] manually verify deploy script works with a chunked bundle
  • [x] ~try out with use-sound library, which seems to trigger a chunked build~
  • [x] try out with import('./local-path').then

Deployed here: https://nameless-fjord-30998.herokuapp.com/

Used this async import:

// establishes socket connection
import('./socket').then(socket => {
  console.log('Got socket!', socket)
})

npm run deploy shows multipart bundle being added:

[deploy 29f0e89] Deploying
 8 files changed, 13998 insertions(+)
 create mode 100644 public/0.bundle.js
 create mode 100644 public/0.bundle.js.map
 create mode 100644 public/1.bundle.js
 create mode 100644 public/1.bundle.js.map
 create mode 100644 public/2.bundle.js
 create mode 100644 public/2.bundle.js.map
 create mode 100644 public/bundle.js
 create mode 100644 public/bundle.js.map