spack list: add --count option (#34950)

Sometimes I just want to know how many packages of a certain type there are.

- [x] add `--count` option to `spack list` that output the number of packages that
      *would* be listed.

```console
> spack list --count
6864
> spack list --count py-
2040
> spack list --count r-
1162
```

share/spack/spack-completion.bash

@@ -1197,7 +1197,7 @@ _spack_license_update_copyright_year() {
 _spack_list() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -d --search-description --format --update -v --virtuals -t --tag"
+        SPACK_COMPREPLY="-h --help -d --search-description --format -v --virtuals -t --tag --count --update"
     else
         _all_packages
     fi