meta: update devcontainer to the latest schema

This PR updates the devcontainer config to that of the latest schema.

The dev container configuration is either located under .devcontainer/devcontainer.json or stored as a .devcontainer.json file (note the dot-prefix) in the root of your project.

IMO .devcontainer.json at the root makes more sense, as it is a similar file to to .editorconfig, which is also stored at the root.

Additionally, extensions and settings where renamed to customizations/vscode/extensions and customizations/vscode/settings, respectively. The latest schema for the vscode customizations is available here.