Add libmem-config.cmake

[nathan818fr]

Following discussions on discord, I'm adding libmem-config.cmake.

Suggesting its use via FetchContent implies carefully maintaining its compatibility over time and not introducing breaking changes.
I suggest creating a dedicated tag, named config-v1 that would track non-breaking updates of this config file.
In this case, the suggested config URL would be: https://raw.githubusercontent.com/rdbo/libmem/config-v1/libmem-config.cmake (instead of relying on the master branch with: https://raw.githubusercontent.com/rdbo/libmem/master/libmem-config.cmake).
And if we ever had to do a breaking change, the config-v1 tag would be frozen, in favor of config-v2, etc.

Also, a version is mentioned in the file (Config version: 2023-12-10.0).
I think this is important because some people will make a copy of the config rather than using FetchContent.
In this case, this will at least let know which exact version was used.

I'm ready to make changes if requested.

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To update an existing tag without affecting other tags: git tag -f config-v1 && git push -f origin config-v1
I mention this because it is often suggested to use git push -f --tags, which can be problematic if other tags have been modified locally.