`prefix` should be exported

[penelopeysm]

Minimal working example

julia> using Turing

julia> prefix
ERROR: UndefVarError: `prefix` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name also exists in DynamicPPL.

Description

Unless there's a reason for us not to (clashes with other packages?), it seems that we forgot to add this to the export list when switching over the submodel stuff.

[penelopeysm]

Closed by #2474 (in Turing 0.37.0)