nano - add missing db.attachment.get signature that returns a ReadableStream (#19634)

* Added a missing db.attachment.get signature

* nano package version number bumped to 6.4 in index.d.ts

types/nano/index.d.ts

@@ -1,4 +1,4 @@
-// Type definitions for nano 6.2
+// Type definitions for nano 6.4
 // Project: https://github.com/apache/couchdb-nano
 // Definitions by: Tim Jacobi <https://github.com/timjacobi>
 //                 Kovács Vince <https://github.com/vincekovacs>
@@ -282,6 +282,7 @@ declare namespace nano {
       params: any,
       callback?: Callback<any>
     ): Request;
+    get(docname: string, attname: string): NodeJS.ReadableStream;
     get(docname: string, attname: string, callback?: Callback<any>): Request;
     get(
       docname: string,

types/nano/nano-tests.ts

@@ -127,8 +127,15 @@ mydb.attachment.insert(
   "text/plain",
   (error: any, att: any) => {}
 );
+const attInsert: NodeJS.WritableStream = mydb.attachment.insert(
+  "new",
+  "att",
+  null,
+  "text/plain"
+);
 mydb.attachment.destroy("new", "att", { rev: "123" }, (err, response) => {});
 mydb.attachment.get("new_string", "att", (error: any, helloWorld: any) => {});
+const attGet: NodeJS.ReadableStream = mydb.attachment.get("new_string", "att");
 
 /*
  * Multipart