Support "uninstalling" packages

[vovacodes]

One of the issues we've run into at Framer is being unable to remove a package from the import map. This causes unnecessary bloat. The API could look something like this:

const generator = new Generator({
        mapUrl: "about:blank",
        inputMap,
})

await generator.uninstall("lodash")

return generator.getMap()

[guybedford]

generator.uninstall is now implemented in #132.