cmd/checksum: allow adding new versions to package (#24532)

This adds super-lazy maintainer mode to `spack checksum`: Instead of
only printing the new checksums to the terminal, `-a` and
`--add-to-package` will add the new checksums to the `package.py` file
and open it in the editor afterwards for final checks.

share/spack/spack-completion.bash

@@ -580,7 +580,7 @@ _spack_change() {
 _spack_checksum() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --keep-stage -b --batch -l --latest -p --preferred"
+        SPACK_COMPREPLY="-h --help --keep-stage -b --batch -l --latest -p --preferred -a --add-to-package"
     else
         _all_packages
     fi