ci(vnext): fail if a shell script is added to the plugin source

[eshwar-sundar-glean]

What

Add a CI guard that fails if a shell/bash script is added under the glean-vnext plugin source, with a message steering contributors to a Node (.mjs) alternative for cross-platform (Windows) compatibility.

The plugin runs on end-user machines including Windows, where POSIX shell is unavailable — we already migrated the launcher from start.sh to start.mjs for this reason. This locks that decision in.

Changes

• scripts/check-no-shell-scripts.mjs — recursive scan of sources/glean-vnext/ for shell extensions (.sh/.bash/.zsh/.ksh) and bash/sh shebangs. A #!/usr/bin/env node shebang (.mjs) passes. Written in Node (not bash) so it runs identically on Windows/macOS/Linux, in CI and locally.
• package.json — new check:no-shell script so contributors can run it locally on any OS.
• .github/workflows/test.yml — runs the guard right after npm ci, before the build, for fast feedback.

On failure, contributors see

❌ Shell/bash scripts are not allowed in the glean-vnext plugin source.

Found 1 shell script(s):
  - sources/glean-vnext/tmp-demo.sh

The plugin runs on end-user machines including Windows, where POSIX shell
is unavailable. Use a cross-platform Node.js script instead:
  - Write it as a .mjs file (see sources/glean-vnext/start.mjs).
  - Use Node built-ins (node:fs, node:path, node:child_process) ...
  - Invoke it with "node <script>.mjs" so it works on Windows/macOS/Linux.

Verification (local)

• ✅ Clean tree → passes (exit 0)
• ✅ .sh file added → fails (exit 1) with suggestion
• ✅ bash-shebang file without .sh extension → fails (exit 1)
• ✅ node-shebang .mjs → not flagged (exit 0)

Scope is intentionally limited to sources/glean-vnext (per request); SCAN_ROOTS in the script can be broadened later.