Overhaul AssemblyNameRegex

`AssemblyNameRegex` in the `FodyWeavers.xml` files should be restricting the assemblies the particular configuration file affects.

Currently the setting just gets merged across files, but it makes more sense to allow setting all settings, including the list of weavers to run for specific assemblies. This allows a library component to use Malimbe (configured with the library's settings) while also allowing any user of that library to use Malimbe with different settings.

Perhaps `AssemblyNameRegex` should move into the root-level element `Weavers` instead? This would need to still allow specifying _multiple_ assemblies, which should be easily possible since it's treated as a regex after all.