Modernize code + dependencies to prepare for React 19

[karmeleon]

This started as "remove defaultProps because React 19 drops support for them" but sort of expanded a bit when a bunch of stuff failed to install.

Runtime changes:

• defaultProps replaced with ES6 default parameters.
• @babel/preset-react now uses the jsx transform instead of React.createElement, since it's also unsupported in React 19.
• @babel/preset-env updated to target > 0.25%, not dead, so features like arrow functions are required at runtime (unless you transpile the module).
• Switched minifier to terser because uglify is no longer supported.

The above changes reduce the tarball size from ~16k to ~12k, and likely include runtime benefits from the new transform, lack of defaultProps (which sound like they carry a penalty), and reduction in polyfills.

Dev environment changes:

• Tons of devDeps upgraded. Webpack, ESLint, Jest, Babel, Flow, Prettier, and React all needed bumping either to support the new features, modern Node, or other upgraded packages.
• Since ESLint and Prettier were upgraded, there were some style changes.
• Snapshots updated, as default destructures don't show up in snapshots like defaultProps did.
• Python venv upgraded to 3.10.
• Node version bumped to v16.
• pre-commit config updated to actually work.
• Travis-CI removed, replaced with Github Actions
  • n.b. the publish step is necessarily untested, but was copied from another Yelp repo so assuming that still works and the npm secret is org-wide, this should work too.

Verification

Tests all pass, and swapping this version in for the current latest version in my local dev environment both renders correctly and quiets all the deprecation warnings under React 18.3.0.

I plan to release this as a major version.