Update parseq-tracevis UI and dependencies (#342)

The trace visualizer UI hasn't been updated in many years. This lack of updates
is catching up with us, since the JavaScript ecosystem moves fast. This change
introduces JavaScript tooling updates to get the trace visualizer at least
somewhat up to date.

The high-level list of changes are:

- Update `nodejs` to 20.8.1
- Update `npm` to 8.15.0
- Update `bootstrap` to v4 (not the latest, but still updated)
- Replace `PhantomJS` with `Puppeteer` for integration testing
- Updated the look and feel of the visualizer slightly due to changes in how
  `bootstrap` works in v4 and partly because the existing UI needs a facelift.
  There is still a lot of room for improvement, but I kept the minimalist
  approach while trying to improve some of the usability and visual alignment of
  things.
  - The different view selectors have icons and correct mouse cursors.
  - The JSON code is now monospaced.
  - Labels and buttons are visually aligned and use the same vertical rhythm.
  - The waterfall view's container has optional horizontal scrollbars rather
    than the entire page.
  - The colors have been tweaked slightly due to changes in `bootstrap`.
  - HTML markup has been updated to remove some of the inlined JavaScript.