fix: add dynamically loaded format via addFormat()

The return value of `loadFormat()` needs to have the same type as the
format you pass in via `addFormat()`. Hence use the `addFormat()` code
path in order to add a new format dynamically.

src/index.js

@@ -455,7 +455,7 @@ class IPLDResolver {
 
     // If not supported, attempt to dynamically load this format
     const format = await this.loadFormat(codec)
-    this.resolvers[codec] = format
+    this.addFormat(format)
     return format
   }