Add node emulation to `typehead serve`

[mbullington]

Description

Parcel bundler has "node integration" which mirrors what Webpack, Browserify, etc do. We wouldn't want to include this in the actual bundle, in case users are using Node.js or a platform we don't recognize, but we can do the following for typescript serve:

1. Prepend a window.global = global; since ESBuild doesn't do this by default.
2. Add the esbuild-plugin-node-emulation plugin.

[mbullington]

This seems like a good candidate: https://github.com/remorses/esbuild-plugins