Mod Compatibility Fixes

[BrickPi]

• Implements a "ModManager" which loads .mod files
• Implements replace_path when searching for files
• Myriad changes to better reflect how V2 handles systems
• HPM now successfully loads and runs under OpenVic

[wvpm]

The changes in this PR do not reflect how Victoria 2 works.
Carefully research Victoria 2 instead of trusting in mods.
Mods often just use things without realising it doesn't work.

[wvpm]

Avoid custom callbacks for invalid keys. Use the default handling instead.
Either map_key_value_invalid_callback or map_key_value_ignore_invalid_callback

[Spartan322]

Everywhere that you use std::vector<T> const& could you instead use std::span<T> as it reduces indirection and generalizes the use case instead of being dependent on the vector type being passed. It would allow any contiguous finite array of elements, dynamic or static, to be passed in and also doesn't depend on an allocator to describe the type.