Typescript bindings for Soroban JS APIs

[sagpatil]

Motivation: Provide first-class TypeScript types for Soroban-related JS APIs so developers get autocompletion and compile-time checks.

Proposed approach:

• Add .d.ts or migrate Soroban JS modules to TypeScript.
• Add example code demonstrating typed contract invocation.
• Add CI typechecks for the bindings.

Acceptance criteria:

• Accurate TypeScript typings for Soroban APIs are available.
• Example usages are present in docs.

[sagpatil]

Parent epic: #1244