[Improvement] Increase flexibility of xmake target settings for mod authors.

[bitonality]

Streamline and document the process of when mod authors want to deviate from the default ue4ss.mod options.

Current def:

-- This rule is meant to be used by mod targets.
-- Logic that should ONLY apply to mods should be defined in this rule.
-- All other common logic is inherited from the ue4ss.base rule.
-- Example:
-- target("ExampleMod")
--   add_rules("ue4ss.mod")
--   add_includeirs(".")
--   add_files("dllmain.cpp")
rule("ue4ss.mod")
    add_deps("ue4ss.base", {order = true})
    after_load(function(target)
        target:set("kind", "shared")
        target:set("languages", "cxx20")
        target:set("exceptions", "cxx")
        target:add("deps", "UE4SS")
        target:set("group", "mods")
    end)

    on_install(function(target)
        import("mods.install").install(target)
    end)

I need to document how mod authors can alter these settings and save their own custom rules if they want. I'm envisioning allowing users to create an extension of ue4ss.mod like ue4ss.mod.cxx_latest or ue4ss.mod.custom_runtime etc. They'll then be able to use their custom rules in any mods in their UE4SS mono-repo.