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/plugin.rs

@@ -1,13 +1,20 @@
 use rbx::RbxItem;
 use vfs::VfsItem;
+use core::Route;
 
-pub enum PluginResult {
+pub enum TransformResult {
     Value(Option<RbxItem>),
     Pass,
 }
 
+pub enum FileChangeResult {
+    MarkChanged(Option<Vec<Route>>),
+    Pass,
+}
+
 pub trait Plugin {
-    fn transform_file(&self, plugins: &PluginChain, vfs_item: &VfsItem) -> PluginResult;
+    fn transform_file(&self, plugins: &PluginChain, vfs_item: &VfsItem) -> TransformResult;
+    fn handle_file_change(&self, route: &Route) -> FileChangeResult;
 }
 
 pub struct PluginChain {
@@ -24,8 +31,8 @@ impl PluginChain {
     pub fn transform_file(&self, vfs_item: &VfsItem) -> Option<RbxItem> {
         for plugin in &self.plugins {
             match plugin.transform_file(self, vfs_item) {
-                PluginResult::Value(rbx_item) => return rbx_item,
-                PluginResult::Pass => {},
+                TransformResult::Value(rbx_item) => return rbx_item,
+                TransformResult::Pass => {},
             }
         }