add basic devcontainer with userspace setup

[pustekuchen91]

In this pull request i want to provide a devcontainer setup. This will help to get started very quick, if vscode + docker/podman is already installed.

Additional vscode task can be provided

Description

Added needed devcontainer files and setup scripts

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[zvecr]

Closing in favour of qmk/qmk_userspace#6