Update to Go 1.20 and React with line numbers

[grantnelson-wf]

I'm doing a lot of experimentation and improvements to the GopherJS playground.

• Updated the README.md
• Added an icon
• Adding CI to address Add some sort of CI to this repo? #47
• Switching to React:
  • Fixes Playground: Consider simplifying by removing go-angularjs dependency. #64
  • Create a React bindings for GopherJS
  • Adding light and dark mode
  • Updating CSS
  • Adding line numbers to fix Playground: Add line numbers #61
  • Made fancy checkboxes
  • Improved dynamic layout
• Adding predefined snippets
• Adding more programmer interactions:
  • Ctrl+/ will comment the selected lines or current line
  • Tab and Shift+Tab will indent and outdent the selected lines
  • Improved automatic indenting
  • Added bracket and other symbol pairing
  • Added hooks for things like Ctrl+S
  • Improving cursor tracking to address Playground: Minor issue, caret is not kept in view when pressing Enter, Tab. #18
  • Fixed undo/redo stack for Format button breaks undo #54
  • Changed editor to use runes to help editing with unicode
    • May fix Playground: Fails to run some code with Unicode. #68
• Updating the compiler
  • Fixed the precompiler to use new form of sources
  • Changed the compiler to use new parsing configuration
  • Made the compiler run in a web worker
    • May fix Playground: Pull out the core so it can be reused #9
  • Made the compiled JS run in a web worker
    • This allows that long processes to be terminated
    • Keeps the playground responsive while code is running
    • Should address Playground: Add sandboxed iframe #5
  • Made compile errors highlight the line numbers next to the code of the error
• Improving Encapsulation for testability and reusability
• and more...

[nevkontakte]

I'll be honest and admit that I gave a cursory look to most of the code here.

Personally, I feel like React is a bit of an overkill for what Playground is, but it's an iprovement over the status quo, and if nothing else a pretty useful tech demo, so I have no real objection.

Thanks!

[nevkontakte]

nit: Shadowing here makes recursion a bit more confusing than necessary.