fix(frontends/lean/decl_cmds): allow private transparent definitions

Example:
section
  universe l
  private definition T := Type.{max 1 l}
  ...
end
feat(emacs): highlight Type'
refactor(library): is_inhabited "theorems" should be "definitions", they are "data"
fix(frontends/lean/decl_cmds): remove assertion that does not hold anymore
fix(frontends/lean/decl_cmds): remove assertion that does not hold anymore