fix: Improve DOM-less type exports (#352)

Usage of HeadersInit requires either a node-fetch dependency or to
include DOM in tsconfig libs for consumer projects. It is not exported
as a global type in @types/node v18 or v20. RequestInit is:

https://github.com/DefinitelyTyped/DefinitelyTyped/blob/aaf94a0a6431f52c46dab26182f1156d79720d29/types/node/v18/globals.d.ts#L341

Define headers in interface HTTPResolverOptions<> via
RequestInit['headers'] which will make it possible to depend on this
project without node-fetch.

lib/types/index.ts

@@ -25,7 +25,7 @@ export interface HTTPResolverOptions<S extends object = JSONSchema> extends Part
   /**
    * You can specify any HTTP headers that should be sent when downloading files. For example, some servers may require you to set the `Accept` or `Referrer` header.
    */
-  headers?: HeadersInit | null;
+  headers?: RequestInit['headers'] | null;
 
   /**
    * The amount of time (in milliseconds) to wait for a response from the server when downloading files. The default is 5 seconds.