fix: take handler for sol_lua_check as function reference

[Nerixyz]

I mentioned this in #6386 (comment) - we shouldn't take the handler as a std::function which could potentially allocate. The handler is called when the check fails, so it's not stored anywhere. Sol passes the handler as a template argument. We could do the same, but this would mean that we'd need to define the functions in the header. Instead, we can use a function reference (like std::function_ref). Since we aren't on C++ 26, we need to add this ourselves. I used llvm::function_ref as the base for this, since it's concise (the actual implementation in libc++ and libstdc++ is larger because they handle more cases like noexcept/const/volatile, but the idea of keeping two pointers is the same). We're using C++ 20, so we can use concepts instead of SFINAE.

[github-actions]

clang-tidy made some suggestions

[pajlada]

Want this to wait until the Link PR is merged in?

[Nerixyz]

Want this to wait until the Link PR is merged in?

I'd be fine with both. Maybe @Mm2PL has a preferred order.

[Mm2PL]

I'm fine with this being merged before link PR is merged, I'll deal with the conflicts. That one still needs a little more time in the oven.