compatibility with create-react-app 5 (webpack 5)

[sheppard]

See wq/wq#58. This seems to be a larger ecosystem issue, so deferring to 2.0 while things are sorted out.

• [x] Fix relative imports to add .js extension
• [x] Fix imports of package internal files from node_modules/
• [x] Handle default export issues
• [ ] Ensure that MUI isn't loaded twice