You can use any client-side JavaScript library stored in a public unpkg module repository and is UMD bundled. You can do it by editing [dependencies].
Let's say you want to import moment library. You can do it by adding its unpkg URL to js.modules. Then in js.imports under moment key which will become a local name of the import add js.imports.moment.source. This must match what's in js.modules and js.imports.moment.importedName is what you want to extract from the module, in this case default.
The below translates to import moment from 'moment' or import { default as moment } from 'moment'