Remove default implementation of getDebugInstance

We have to make a judgement call per occurance to determine what we
should return here.

This whole mess could use some simplification, but we probably just
won't end up keeping this around for much longer once/if we remove
the action modifier, which is currently the only kind of modifier
we have internally (since `{{on}}` is implemented in glimmer-vm and
essentially duplicates this whole "internal" set up once more time
over there).

packages/@ember/-internals/glimmer/lib/modifiers/internal.ts

@@ -52,7 +52,7 @@ class InternalModifierState implements Destroyable {
   constructor(readonly instance: InternalModifier) {}
 }
 
-export class InternalModifierManager
+export abstract class InternalModifierManager
   implements ModifierManager<InternalModifierState, typeof InternalModifier>
 {
   constructor(private ModifierClass: typeof InternalModifier, private name: string) {}
@@ -78,9 +78,7 @@ export class InternalModifierManager
     return null;
   }
 
-  getDebugInstance(): unknown {
-    return null;
-  }
+  abstract getDebugInstance(state: InternalModifierState): unknown;
 
   getDebugName(): string {
     return this.name;