Add basic Imenu support for easy navigation.

[carltonf]

No description provided.

[omajid]

Some version of this would be great to have. Navigation by sections just makes so much sense in longer spec files. I have an alternate implementation at omajid@dde79cd.

[Thaodan]

The PR uses (easy-menu-add) however it is obsolete since Emacs 28.1
and doesn't do anything.

See their note:

This function is obsolete since 28.1; this was always a no-op in Emacs
and can be safely removed.

[Thaodan]

The patch also uses (add-to-list) for rpm-imenu-index however this doesn't work.

See Emacs output here:

lib/rpm-spec-mode/rpm-spec-mode.el:682:8: Error: ‘add-to-list’ can’t use lexical var ‘rpm-imenu-index’; use ‘push’ or ‘cl-pushnew’

[Thaodan]

See here for fixes for those issues:
https://github.com/Thaodan/rpm-spec-mode/tree/add-imenu-support-fixes