fix: Replace static mut with Lazy for thread safety in FileBoxRegistry

- Eliminate static_mut_refs warnings by using once_cell::sync::Lazy
- Make FileMode enum public to fix private_interfaces warning
- Reduce warnings from 6 to 3
- Prepare for Rust 2024 edition compatibility

src/bid/loader.rs

@@ -1,7 +1,6 @@
 use crate::bid::{BidError, BidResult, NyashHostVtable, NyashPluginInfo, PluginHandle, PLUGIN_ABI_SYMBOL, PLUGIN_INIT_SYMBOL, PLUGIN_INVOKE_SYMBOL, PLUGIN_SHUTDOWN_SYMBOL};
 #[cfg(all(feature = "plugins", not(target_arch = "wasm32")))]
 use libloading::{Library, Symbol};
-use std::ffi::c_void;
 use std::path::{Path, PathBuf};
 
 /// Loaded plugin with FFI entry points and metadata