The API is so good that a lot of core VS Code behavior (e.g. Github integration, support for lots of languages) is implemented in the form of built-in extensions.

It is possible to get 80% of VS Code's functionality with 10-20% of the code if you just bake everything into one monolith, but this has been tried repeatedly and it keeps failing in part because the extension ecosystem and attendant network effects form a wide moat.

(disclaimer - I work on Positron)

(But that’s just the editor component, if you need all the other IDE stuff you’ll have to build it :-)

For something non-browser, I’m currently using Zed and it’s pretty good: https://zed.dev/