Some build fixes for the master (and gnome-46) branches for Visual Studio

Hi,

This attempts to fix some things that Visual Studio did not like which crept up in the previous cycle:

• Usage of designated initializers in gi/object.cpp, as this is considered only accepted in C++-20, so use accessors to set things up there.
• The GjsDeprecationMessageId enumeration in gjs/deprecation.h was not explicit enough for calling unsigned{x} (or the like) on it, so specify an explicit unsigned base type for it.

With blessings, thank you!