main: Move MetaCompositorType to a new meta-enums.h

It'll be part of and owned by MetaContext, intending to replace
`meta_is_wayland_compositor()`, but place it in a new file for public
enums so that it can be used from wherever.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/tests/stage-view-tests.c

@@ -22,6 +22,7 @@
 #include "compositor/meta-plugin-manager.h"
 #include "core/main-private.h"
 #include "meta/main.h"
+#include "meta/meta-enums.h"
 #include "tests/meta-backend-test.h"
 #include "tests/monitor-test-utils.h"
 #include "tests/test-utils.h"