Review debug mode management in the meson template

We can't just rely on the buildtype option to decide which flags to add, as the user can directly set the debug and optimization options, which usually result in a buildtype option set to custom. It also seems that buildtype=debugoptimized corresponds to a production build, for which we shouldn't enable debug logs (see reference below).

This doesn't exactly reproduce the autotools build mode of the --enable-debug option, which we should probably accept having to deviate from:

• --enable-debug={full,yes} are mixed up in the same debug mode, where all debug logs are enabled (which is perhaps no bad thing?). This corresponds to buildtype=debug.
• --enable-debug=minimal corresponds to buildtype=debugoptimized, where no flag is set.
• --enable-debug=no corresponds to buildtype={minsize,release}, where we set aggressive optimization flags.
• buildtype=plain is also a production mode, for which we don't set debug flags, but for which we don't really know the optimization mode, so we don't set aggressive optimization flags either.

Related: !98 (merged)
See-also:
https://blogs.gnome.org/mcatanzaro/2022/07/15/best-practices-for-build-options/
https://mesonbuild.com/Builtin-options.html