chore: create devcontainer config

[TristanWasTaken]

Create devcontainer config.

It's used by gh's codespaces, VSCodes's devcontainer extensions and allows you to move your dev env to your local or an external docker host.

The config won't need any maintenance until Debian 12 (bookworm) or Node.js version 20 is deprecated, which would be in 2026. While the VSCode extensions might break or become incompatible, it will only affect the extension.

vscode:

• tutorial: Developing inside a Container, Dev Containers tutorial
• extension: Dev Containers

I added a few VSCode extensions for convenience. Please tell me if I should remove any (or all) of them:

• protobuf (proto language support)
• GitHub Actions (config, etc support. added it since the repo has gh action configs)
• Error Lens (highlights & shows errors)
• TODO Highlight (highlights TODO:)

[ronny1982]

I don't see much benefit for the following reasons:

• codespaces/devcontainers are probably not commonly used by HakuNeko developers
• codespaces cannot run electron or nw since an X server is not running
• codespaces can be started via GitHub in browser without the need of a configuration file

[TristanWasTaken]

fair. I wasn't really sure myself if it's worth it (which is why I asked on Discord beforehand)

* codespaces can be started via GitHub in browser without the need of a configuration file

I didn't know that it defaults to the node image and thought it's a barebone install. Kinda cursed, but you learn something new every day 😄