[GSoC 2025] Templated Hooks - Weekly Update #5

[AlbertG]

This week I've worked on the templatization of 
[`_adEq2`](https://github.com/dlang/dmd/pull/21513), more 
precisely replacing all `_adEq2` lowering with the `__equals` 
templated hook. On top of that, I have refactored the code by 
adding a new `.lowering` field which now stores the lowering 
instead of replacing the original `EqualExp` with the lowerred 
expression. There are still some bugs and optimizations I have to 
look over.