deprecation: Add deprecation warning framework
This adds a framework for deprecation warnings. It consists of an enum of deprecated features, and the API _gjs_warn_deprecated_once_per_callsite() which warns only once in each place the feature is used, even if that code is executed more than once. This is meant to be joined in the future by _gjs_warn_deprecated_once() and _gjs_warn_deprecated_once_per_file(), since we have unconditional deprecation warnings elsewhere in GJS that could very well use those APIs.
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