[typespec-ts] JavaScript — Support Post‑Emit Execution Step in JS Emitter

## Context
We got the ask from Foundry team that JS SDK generation requires post‑generation scripts to update the generated code.
e.g. update code emitter outputs and to deal with emitted code issues that service team hasn't decided the correct approach for yet.

Currently, these are executed manually which is fragile and inconsistent.

## Problems with Current State

- JS pipelines may need to run adhoc custom PowerShell scripts after emission
- No standardized way for JS emitter to trigger these steps

## Proposal

Add post‑emit execution support in the JS emitter
The emitter should invoke a script after code emission but before downstream pipeline steps