Replace Webpack dev server with zero-dependency alternative

[aduth]

🛠 Summary of changes

Implements a lightweight, zero-dependency alternative to webpack-dev-server.

Why?

webpack-dev-server is pretty feature rich, but we use very few of its features. Arguably, we could get rid of it altogether and everything would be fine, but some features like pausing the request while compilation is in-progress are useful to improve confidence during development when testing changes.

Of note is that webpack-dev-server creates a very deep dependency tree because it's implemented as an Express-based web server. Moving away from it has additional benefits:

• Decreases risk and maintenance burden for security advisories. Express and related dependencies are frequent targets for security advisories, and consequently these security advisories bubble up through webpack-dev-server. All 9 of the 9 open security advisories trace back to webpack-dev-server.
• Decreases inherent risk and installation time for third-party dependencies. Dropping webpack-dev-server drops 133 dependencies (!!!)†, and reduces the size of yarn.lock by 996 lines (-40kb / -12%).

† Counted via: git diff main -- yarn.lock | tail -n +5 | grep -E "^-\S" | wc -l minus git diff main -- yarn.lock | tail -n +5 | grep -E "^\+\S" | wc -l

📜 Testing Plan

Verify that JavaScript continues to compile and behave as expected in local development:

1. rm -rf public/packs
2. make run
3. Go to http://localhost:3000
4. Verify that JavaScript behaviors still work (e.g. "Here's how you know" banner collapse, language selector, field validation, etc).

[zachmargolis]

LGTM big fan! small questions, not blockers