meson: added variants, changed defaults for the build system (#22715)

- Use debugoptimized as default build type, just like RelWithDebInfo for cmake
- Do not strip by default, and add a default_library variant which conveniently support both shared and static

lib/spack/docs/build_systems/mesonpackage.rst

@@ -121,11 +121,15 @@ override the ``meson_args`` method like so:
 .. code-block:: python
 
    def meson_args(self):
-       return ['--default-library=both']
+       return ['--warnlevel=3']
 
 
 This method can be used to pass flags as well as variables.
 
+Note that the ``MesonPackage`` base class already defines variants for
+``buildtype``, ``default_library`` and ``strip``, which are mapped to default
+Meson arguments, meaning that you don't have to specify these.
+
 ^^^^^^^^^^^^^^^^^^^^^^
 External documentation
 ^^^^^^^^^^^^^^^^^^^^^^