Fix JS scaffold build + add CLI template smoke test

[jodeleeuw]

Summary

A user hit babel: command not found running npm run build on a freshly scaffolded new-plugin JS package. Root cause was a broken JavaScript template, and validating the fix surfaced a second build bug — so this PR fixes both across all three generators and adds CI to keep it from regressing.

Template fixes (all three JS templates)

• Missing Babel toolchain: the build script invokes babel with @babel/preset-env + the minify preset, but none of @babel/cli, @babel/core, @babel/preset-env, babel-preset-minify were in devDependencies. After a clean npm install the babel binary didn't exist → "babel not found". Pinned to the Babel 7 line, which is compatible with the (2022-era) babel-preset-minify.
• Wrong source path: build compiled index.js at the package root, but the source ships at src/index.js (like the TS templates) → babel: index.js does not exist. Fixed the path and updated files to publish src instead of the non-existent index.js.

Affects @jspsych/new-plugin, @jspsych/new-extension, @jspsych/new-timeline (patch changeset included). TS templates were already fine.

New CI: CLI Template Smoke Test

End-to-end workflow that, for a 3×2 matrix (each CLI × ts/js), scaffolds a package non-interactively, then runs npm install + npm run build in the generated package and asserts dist/index.browser.min.js is produced. This is what caught the second bug. Runs on PRs touching the CLI packages, pushes to main, and manual dispatch.

Testing

Ran the full scaffold → install → build flow locally for all six combinations — all pass:

new-plugin     ts/js -> plugin-smoke-test      OK
new-extension  ts/js -> extension-smoke-test   OK
new-timeline   ts/js -> smoke-test             OK

Note (out of scope)

package-lock.json is out of sync — npm ci fails on missing @esbuild/* platform optional deps (pre-existing, also affects release.yml). Happy to regenerate the lockfile in a follow-up.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: b4fcb5b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@jspsych/new-plugin	Patch
@jspsych/new-extension	Patch
@jspsych/new-timeline	Patch

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