Pack server code together using webpack.

[yuchenshi]

To create a zipball:

npm run build:server
zip dist.zip -r server.bundle.js build

# Now try to extract it somewhere else and see if it runs:
cd /tmp/
unzip /path/to/ui/dist.zip
firebase emulators:exec --project foo --only database 'node server.bundle.js'

Now you can visit localhost:3000 for a bundled version of the Emulator UI.

Note that this is not gonna work later if we ever introduce native dependencies (e.g. grpc, or anything that depends on grpc like Firestore SDK.)

[samtstern]

Tested building on Linux and using the bundle on both Linux and macOS