Add handle_file_change to plugin API

This solves the problem I was running into with the ScriptPlugin implementation -- if foo/init.lua changes, foo needs to be requested, not 'foo/init.lua' (which would then erroneously create a ModuleScript before this commit)

src/core.rs

@@ -4,3 +4,5 @@ pub struct Config {
     pub verbose: bool,
     pub server_id: u64,
 }
+
+pub type Route = Vec<String>;