Add VS Code RTF Preview extension to book and devcontainer #76
Description
We should recommend the new VS Code extension for previewing RTF files built by @elong0527 as part of the developer toolchain: https://marketplace.visualstudio.com/items?itemName=elong0527.vs-rtf-preview
- Metioned in the VS Code extension section: https://pycsr.org/env-dev.html#vs-code
- Add the extension to devcontainer so RTF can be directly previewed when clicked in Codespaces.
- Also add to the devcontainer of the esub package repo: https://github.com/elong0527/demo-py-esub