feat(defaults): mouse=nvi #19290

Problem:
Since right-click can now show a popup menu, we can provide messaging to
guide users who expect 'mouse' to be disabled by default. So 'mouse' can
now be enabled by default.

Solution:
Do it.
Closes #15521

src/nvim/main.c

@@ -356,7 +356,14 @@ int main(int argc, char **argv)
     abort();  // unreachable
   }
 
-  init_default_mappings();  // Default mappings.
+  // Default mappings (incl. menus)
+  Error err = ERROR_INIT;
+  Object o = nlua_exec(STATIC_CSTR_AS_STRING("return vim._init_default_mappings()"),
+                       (Array)ARRAY_DICT_INIT, &err);
+  assert(!ERROR_SET(&err));
+  api_clear_error(&err);
+  assert(o.type == kObjectTypeNil);
+  api_free_object(o);
   TIME_MSG("init default mappings");
 
   init_default_autocmds();