Fix some typos, small errors, debug leftovers. Removed unused
mu-cmd-count.cc. Improve test-coverage. Fix manpage.
Make the errors for unknown commands a little friendlier.
Move the various seq_ functions, as well as std::(stable_)sort,
std::accumulate, std::transform, std::find, std::find_if to their C++20
std::ranges counterparts.
Support "system" properties, i.e., library versions, whether
language-detection, scm is supported.
Make these more generic (with display strings), so we can handle them
more automatic int mu-cmd-info.
Only include xapian.h in one place, so we can have consistent options.
With that in place, we can enable C++ move semantics.
We don't do anything with that yet, but we check in the meson.build file
to see if we have the required xapian version.
Xapian supports an "ngrams" option to help with languages/scripts
without explicit wordbreaks, such as Chinese / Japanese / Korean.
Add some plumbing for supporting this in mu as well. Experimental for
now.
