Add support for .custom.hx handling

[kLabz]

Note that this couldn't be implemented as a define or a macro API because it needs to be set before running init macros.

Closes #10767

[Simn]

I almost don't want to ask, but what about .custom.target.hx and .target.custom.hx?

[kLabz]

I almost don't want to ask, but what about .custom.target.hx and .target.custom.hx?

Juraj came to the same conclusion as me in #10767 : it's likely best not to open that can of worms 😅

[back2dos]

I wonder if it makes sense to allow multiple of these things. But in any case, really nice to see this ;)