[TS] Make strict compliant and improve typings (#7549)

* [TS] Make strict compliant and improve typings

* clang-format

* Code gen harmonize

Co-authored-by: Derek Bailey <derekbailey@google.com>

ts/types.ts

@@ -10,5 +10,8 @@ export type Table = {
 
 export interface IGeneratedObject {
   pack(builder:Builder): Offset
-  unpack(): IGeneratedObject
+}
+
+export interface IUnpackableObject<T> {
+  unpack(): T
 }