build: ensure npm is used for watch task

[ctlai95]

Problem

 *  Executing task in folder amazonq: yarn run watch 

zsh:1: command not found: yarn

 *  The terminal process "/opt/homebrew/bin/zsh '-l', '-c', 'yarn run watch'" failed to launch (exit code: 127). 
 *  Terminal will be reused by tasks, press any key to close it. 

Seems to be a vscode issue.

• NPM script runs with yarn instead of npm. microsoft/vscode#181006
• VSCode uses yarn to run npm tasks microsoft/vscode#159465

Given that we already have npm.packageManager set to npm in this repo, I'm not sure why it keeps trying to use yarn.

Solution

Run the watch task as a shell script

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jpinkney-aws]

TY for making this change. I had been using:

"npm.packageManager": "npm",
"npm.scriptRunner": "npm",

but solving at the tasks level is probably a lot better