allow symbol as an event type

This was hiding because I have 'skipLibCheck' in most of my packages,
since it makes TS builds so much faster, and it's typically not a
problem, since libs get checked in their own development anyway.  But in
the case of something like Minipass, where it's not *actually*
typescript, but just JavaScript with a hand-written .d.ts, things can go
unnoticed.

Still an open todo to migrate minipass to a TS hybrid build, but in the
meantime, this fixes the problem for node-glob.

Fix: isaacs/node-glob#529
Fix: #49
Close: #50

index.d.ts

@@ -69,7 +69,7 @@ export namespace Minipass {
     : ObjectModeOptions
 
   interface EventArguments {
-    [k: string]: unknown[]
+    [k: string | symbol]: unknown[]
   }
   /**
    * The listing of events that a Minipass class can emit.