Change Vortex Mod ID to string.

It doesn't have to be a number, it can have values like "skse64" for non-Nexus mods.

This also changes the types in the Vortex extension, but since this has no effect on the generated JS, it shouldn't require a new extension release.

Fixes #30.

Focus.ModManagers.Vortex.Launcher/src/index.ts

@@ -10,12 +10,12 @@ enum GameId {
 }
 
 interface IModAttributes {
-  modId: number;
+  modId: string;
   modName: string;
 }
 
 interface IFileInfo {
-  modId: number;
+  modId: string;
 }
 
 interface IModInfo {

Focus.ModManagers.Vortex/ModManifest.cs

@@ -6,13 +6,13 @@ namespace Focus.ModManagers.Vortex
     public class ModManifest
     {
         public Dictionary<string, FileInfo> Files { get; set; }
-        public Dictionary<int, ModInfo> Mods { get; set; }
+        public Dictionary<string, ModInfo> Mods { get; set; }
         public string StagingDir { get; set; }
     }
 
     public class FileInfo
     {
-        public int ModId { get; set; }
+        public string ModId { get; set; }
     }
 
     public class ModInfo