feat!: move mime-db to peerDependencies

[broofa]

See prior conversation at #114

[dougwilson]

I think the main part the convo left off is how to work through dealing with multiple mime-types majors being in the dep tree at the same time and them wanting different mime-db majors.

So like we'd need to have mime-types support at minimum one major mime-db version prior each time or have a delicate dance of getting everything updated all in order. I would guess that express would always need form-data, then superagent, and finally supertest to all update their mime-types major before express itself could if multuple mime-db majors are not supported by mime-types.

[ljharb]

A challenge is that if mime-types depends on mime-db with "v1 or v2", eg, then npm's auto-installing logic will install v2 by default, so v1 users would be forced to be explicit with their dependency - which could arguably be considered a breaking change to v1 users.

[broofa]

Yeah, that's kind of where my head went to. mime-types@next would have to be [email protected] aware (declare something like "mime-db": ">=1.0.0 <3.0.0 in its peerDependencies), to allow downstream modules to update without being blocked by upstream dependencies.