test-fixtures: modernize & standardize

[msimerson]

• [x] minimize package size when publishing. Use [files] in package.json instead of .npmignore.
• [x] automated code linting. This is mostly done already via the lint GHA workflow.
• [x] automated code formatting. Eslint is exiting this game. Prettier is the 900# gorilla.
• [x] consistent naming of "special" files like CHANGELOG.md.
• [x] automated CI testing. This is mostly done, verify that ci.yml is up-to-date.
• [x] CONTRIBUTORS: some automated means for generating and maintaining that info in the repo.
  • in contributors in package.json?
  • a file CONTRIBUTORS.md
  • filters out bots
• [x] CHANGELOG: verify links at bottom (due to inconsistent tag naming)
• [x] remove eslint from devDependencies in package.json
  • use npx eslint@8 ... syntax instead (installs as needed)
  • suggestion: install eslint globally, saves dev time
• [x] verify GitHub repo About link points to npm package