[JS/TS] Modernize TypeScript / JavaScript flatbuffers support (#6095)

ts/types.ts

@@ -0,0 +1,14 @@
+import { ByteBuffer } from './byte-buffer'
+import { Builder } from './builder'
+
+export type Offset = number;
+
+export type Table = {
+  bb: ByteBuffer
+  bb_pos: number
+};
+
+export interface IGeneratedObject {
+  pack(builder:Builder): Offset
+  unpack(): IGeneratedObject
+}