Add a `.devcontainer` manifest

This could be useful for quickly spinning up a dev environment, e.g. in a local container or a codespace:

- https://code.visualstudio.com/docs/devcontainers/create-dev-container
- https://github.com/microsoft/vscode/tree/main/.devcontainer (an example)

The setup should follow the standard Ubuntu setup:

- Run `Scripts/install-dependencies-apt` or install the equivalent dependencies manually
- Download Swift from https://www.swift.org/download and place it e.g. in `/opt/swift`
- Add symlinks to make it accessible from the `PATH`:

```sh
ln -s /opt/swift/usr/bin/swift /usr/local/bin/swift
ln -s /opt/swift/usr/bin/sourcekit-lsp /usr/local/bin/sourcekit-lsp
```

The only stumbling block is that the default image in GitHub Codespaces is currently based on Ubuntu 20.04, which doesn't have `libstdc++-12-dev`. However it seems like the installed `libstdc++-9-dev` should be fine too.

Alternatively we could of course try use something based on the `swift`-Docker image, similar to our production image (though of course without the slimmed down runner image or cross-compilation).

Start by reading Scripts/install-dependencies-apt and inspect the existing production Docker image referenced in the issue. Add a .devcontainer manifest that follows the Ubuntu setup, installs Swift and the required symlinks, and works in a local container or Codespace. Verify that the dependency installation and Swift tooling are available in the resulting environment.

Written by the indexing model from the issue text.