Assumption reference-able theorem-like environment

[i-thot-u-wanted2dance]

It would be great to add "Assumption" to the list of supported reference-able theorem-like environments.

Some candidates for a suitable label prefix:

• #asm-
• #asp-
• #asu-

[mcanouil]

I am closing this since it is already been tracked on #3979 (initially from #3970)

[cscheid]

Feel free to refer directly to the epic issue as well.