Use VSCode Monaco code editor

[Peter-Juhasz]

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• There is a tremendous number of issues with the existing code viewer:
  • accuracy of syntax highlighting
  • there is no real symbol search, only text search (even though with the new search engine it is marketed as symbol search)
  • consistency of code folding
• The current code editor (by clicking Edit on a file) has effectively zero features (other than a broken syntax highlighting, some brace matching and indentation)

Replace GitHub's classic code viewer (including diff view) & editor component with Visual Studio Code's Monaco Editor - "The Editor of the Web".

• It is much more powerful and has rich support for many languages
• Since Visual Studio and GitHub is both part of Microsoft, and the maintenance cost of a code editor (including viewing code) is very high (because of the both the complexity of such a component and the many supported languages), resource cost could be rationalized by having to maintain only one component going forward, instead of two
• Community support is already there for extensions (including language support), which could further reduce maintenance cost, and would also make new features available
• It is extensible, so GitHub could customize it to its needs
• Its backend supports remote workspaces already (or just simply the LSP can work remotely), so it could either work in the browser or remotely on the backend of GitHub (like github.dev) for an even better experience (language services that can't run in the browser)