feat(frontends/lean): add 'universe variable' command

We can declare variables anywhere. So, we must also be able do declare
"universe" variables anywhere. Here is a minimal example that requires
this feature

```
-- We want A and B to be in the same universe
universe variable l
variable A : Type.{l}
variable B : Type.{l}
definition tst := A = B
```

The following doesn't work because A and B are in different universes
```
variable A : Type
variable B : Type
definition tst := A = B
```

The following works, but tst is not universe polymorphic, since l is
one *fixed* global universe
```
universe l
variable A : Type.{l}
variable B : Type.{l}
definition tst := A = B
```