Add `GC.monitor` functionality
D1 runtime provides GC monitor callbacks (https://github.com/sociomantic-tsunami/tangort/blob/32ad03f41da038b6e315931c551ddfe4cd0d26eb/src/core/memory.d#L478-L479) to run custom hooks on collection begin/end. Some of our apps use it for tracing/stats.
We need to propose similar functionality to upstream druntime and also add support to dmd-transitional. Foruntately it is not a language change as should work as a plain PR with no DIP.