Update flake and playground

[deemp]

• Removed miso
• Switched to CodeMirror 6 + NextJS (closes Update playground to CodeMirror 6 #11)
• Updated compiler version: 9.2.7 -> 9.6.2
• Updated nixpkgs in flake inputs
• Added default rzk package
• Removed rzk.nix
• Added hie.yaml for Haskell Language Server to not complain about missing modules in rzk
• Updated cache-nix-action: v1 -> v4
  • enabled garbage collection
  • enabled purging caches created more than 4 days ago after saving a new cache
• Refactored flake.nix into several .nix files
• Updated development instructions in README
• Updated and formatted README using markdown-all-in-one
• Unsuccessfully experimented
  • tried GHC 9.6.2 with JS backend to achieve incremental builds
  • tried GHCJS 8.10.7
  • tried JSaddle. Abandoned in favor of NextJS and next dev
• Added Lezer grammar and initial styling.
  • followed docs on a Language package (link)
  • defined colors (link)
  • defined grammar (link)
  • defined mapping between parse tree nodes (uppercase words in the grammar) and tag types (link).

Editor

https://deemp.github.io/rzk/develop/playground/

It's resizable. Click and drag the line between the editor and output panel.

[deemp]

Don't need rzk.nix anymore.

[fizruk]

@deemp can you bring this branch up-to-date?

[deemp]

Made new commits for more or less logically connected changes. Squashing was inconvenient.

Rebased on rzk/develop