TS Bindings: Default exports in addition to exporting namespaces

[piyalbasu]

Could be helpful to include default export in the generated library.

This would allow users to do:

import HelloContract from "bindings/hello-soroban-client";
import IncrementContract from "bindings/incrementor-client";

Instead of

import * as HelloContract from "bindings/hello-soroban-client";
import * as IncrementContract from "bindings/incrementor-client";

Useful when dealing with multiple contract namespaces in one file