sh: Add distrobox alias and enable displaying host

Add a simple alias to quickly use distrobox with `db` alias, and modify
pure prompt initialization slightly so that it displays a hostname when
in a distrobox container (akin to operating from ssh or normal container
usage).

HACKY implementation makes use of both an internal pure prompt api
(see here https://github.com/sindresorhus/pure/issues/585)
and a distrobox env var that I am not sure how exposed it is either.
Powerlevel10k uses a similar method though, so maybe it is fine
(see here 33916e91a7/internal/p10k.zsh (L8336)).

sh/.config/sh/alias

@@ -119,3 +119,8 @@ elif exist ipython; then
 elif exist python; then
     alias py=python
 fi
+
+# distrobox
+if exist distrobox; then
+    alias db=distrobox
+fi