Use esbuild to produce a single file

[ggrossetie]

• Upgrade target from es6 to es2020 (to reduce bundle size)
• Upgrade minimum VS code version to 1.60.0

TODO

• make sure that localization is still working when using a single file

resolves #491

[ggrossetie]

Currently, the bundle size is 2.8mb.
I didn't notice any performance improvement on desktop but esbuild is pretty fast and hopefully the tree shaking will eventually decrease the bundle size.

We might also want to use esbuild to bundle the web extension (in a follow-up pull request) as, currently, Webpack produces a 3.6mb bundle. Alternatively, we could try to tune Webpack. For reference, the current configuration is heavily inspired by: https://github.com/microsoft/vscode/blob/main/extensions/markdown-language-features/extension.webpack.config.js