mu: add zsh completion

Remove the ancient zsh completion, and replace with a new, more complete
one. Automatically install (but configuration through meson options)

mu/completion/meson.build

@@ -0,0 +1,29 @@
+## Copyright (C) 2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 3 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; if not, write to the Free Software Foundation,
+## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+
+
+if get_option('zsh-completion').allowed()
+
+  if get_option('zsh-completion-dir') == ''
+    zcdir = datadir / 'zsh' / 'site-functions'
+  else
+    zcdir = get_option('zsh-completion-dir')
+  endif
+
+  install_data('mu-completion.zsh',
+               rename: '_mu',
+               install_dir: zcdir)
+endif