Remove esm dependency

[okwolf]

Building on #20, perhaps we should consider an approach for dropping to need to have any dependencies. Possible strategies, roughly in order of preference/effort:

• Use the esm package as a submodule and publish both together
• Use the published esm package and have a script to pull the latest from npm periodically
• Port the minimal subset of the esm code to support what's needed (how to maintain?)
• Write new code to support ES modules from scratch

This will remove the need to have local install in order for the import used by coverage to work.
Perhaps consider switching from a dependency to a submodule?