Allow extension authors to add configuration files to chokidar's watchlist

[cwstra]

For a typical GraphQL codegen workflow, there tends to be two files that control most of the action for the overall project:

• a GQL schema file, and
• a configuration file for codegen itself.

When using an embed, it would be convenient for a CLI to watch those files, update the config on changes, and re-generate the Rescript outputs. Since the embed lang already spins up chokidar, it seems like we should be able to add some way to allow embed creators to tap into the watcher without overmuch hassle.

[cwstra]

(I might take a stab at this myself tomorrow, but thought I should get the idea down before getting some shut-eye)