Add/standardize Next and Remix demos

[merrywhether]

Following up on https://github.com/electerious/use-ackee/issues/8#issuecomment-1003597882, I was inspired to add a Next.js demo. While I was at it, I also added a Remix demo and updated the React Router demo to CRA v5 and react-router v6.

Working across the 3 apps, I then tried to standardize the various directory structures and READMEs for better predictability/comparability:

• page components are all identical and nav extracted (these files can now be easily copy-pasted into new demos)
• hoisted demo .gitignore entries to minimuze clutter in the demos themselves (also deleted optional files like jsconfig.json where possible for the same reason)
• READMEs link to relevant file showcasing usage

Also attempted to reach a uniform code style based on your lint rules and other code (function expressions over declarations, 2 import groups, etc) without adding linting. Each of these frameworks would like to do their own linting which made things a little weird (CRA could see the root package.json's eslintConfig for instance). Added a .prettierrc with the 2 major settings to align with your established patterns and make things easier for those with Prettier configured, but happy to remove it as well.

Next app running:

Remix app running:

Updated CRA+RR app running: