use @jspm/generator for importmap

[mdynnl]

@jspm/generator still uses node crypto even if integrity isn't supplied so currently added polyfill. We don't need integrity here so, the alternatives are

1. fix it upstream
2. patch (pnpm patch) getIntegrity https://github.com/jspm/generator/blob/e922bda037c62b3a8b86351fc08e6269dde3c83e/src/common/integrity.ts
3. polyfil crypto with empty mocks

Not really sure what we should be doing about existing importmap though. Feel free to suggest anything or take over.