Retire deprecated Bluebird dependency

[tommi-leino-fidera]

Suggestion

Can we retire the Bluebird dependency? It is in practice deprecated:
https://github.com/petkaantonov/bluebird/
petkaantonov/bluebird#1684

[achrinza]

I'd like to propose this change as part of a TypeScript rewrite & package fix up.

I'll see if I can prepare a draft PR in the upcoming weeks for a "v6 alpha" to test the waters.

Loosely-related: loopbackio/loopback-datasource-juggler#1904