Establish simple CLI for serving user code

[rekmarks]

Following #229, we will be able to start vats with different user code. In order to actually do this during development / demo time, we will want to build and serve user code locally. We should establish a simple CLI, a Yargs application modeled on @metamask/snaps-cli, in order to do this.

The CLI should have two commands to start with:

• bundle, for bundling vat user code
  • This should use the endo bundler referenced in Give each vat its own personality #229
• serve, for serving a bundle
  • The server implementation can be modeled on the one from the Snaps CLI