Add VSCode/Codespaces Dev Container settings (#687)

### Description
This adds `.devcontainer/devcontainer.json` so that VSCode will
automatically download and start a Docker container and load a
development environment for Quokka inside this container.

For more information, see:
https://code.visualstudio.com/docs/devcontainers/containers#_quick-start-open-a-git-repository-or-github-pr-in-an-isolated-container-volume.

This will also enable GitHub Codespaces:
https://docs.github.com/en/codespaces/overview#what-is-a-codespace.

### Related issues
N/A

### Checklist
_Before this pull request can be reviewed, all of these tasks should be
completed. Denote completed tasks with an `x` inside the square brackets
`[ ]` in the Markdown source below:_
- [x] I have added a description (see above).
- [ ] I have added a link to any related issues see (see above).
- [x] I have read the [Contributing
Guide](https://github.com/quokka-astro/quokka/blob/development/CONTRIBUTING.md).
- [ ] I have added tests for any new physics that this PR adds to the
code.
- [ ] I have tested this PR on my local computer and all tests pass.
- [ ] I have manually triggered the GPU tests with the magic comment
`/azp run`.
- [x] I have requested a reviewer for this PR.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

.devcontainer/Dockerfile

@@ -0,0 +1,23 @@
+FROM mcr.microsoft.com/devcontainers/cpp:ubuntu-24.04
+
+RUN apt-get --yes -qq update \
+ && apt-get --yes -qq upgrade \
+ && apt-get --yes -qq install build-essential \
+                      git cmake clangd gcc g++ \
+                      python3-dev python3-numpy python3-matplotlib python3-pip \
+                      sphinx-doc python3-sphinx-rtd-theme python3-sphinxcontrib.bibtex python3-sphinx-copybutton \
+                      libopenmpi-dev \
+                      libhdf5-mpi-dev \
+ && apt-get --yes -qq clean \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /home/ubuntu
+USER ubuntu
+
+# install esbonio for Sphinx VSCode support (no Ubuntu package for this)
+RUN pip install esbonio --break-system-packages
+
+# workaround Python babel bug
+ENV TZ=UTC
+
+CMD [ "/bin/bash" ]

.devcontainer/devcontainer.json

@@ -0,0 +1,35 @@
+// devcontainer.json
+    {
+      "name": "quokka-dev",
+      // (NOTE: using the cloud image is temporarily disabled, since it doesn't exist yet)
+      // "image": "ghcr.io/quokka-astro/quokka:development",
+      "build": {
+        // Path is relative to the devcontainer.json file.
+        "dockerfile": "Dockerfile"
+      },
+      "hostRequirements": {
+        "cpus": 4
+      },
+      "customizations": {
+        "vscode": {
+          "settings": {},
+          "extensions": [
+            // disabled, since it interferes with clangd
+            "-ms-vscode.cpptools",
+            "llvm-vs-code-extensions.vscode-clangd",
+            "github.vscode-pull-request-github",
+            "ms-python.python",
+            "ms-toolsai.jupyter",
+            "ms-vscode.live-server",
+            "ms-azuretools.vscode-docker",
+            "swyddfa.esbonio",
+            "tomoki1207.pdf" // this extension does not work on codespaces
+          ]
+        }
+      },
+      "remoteUser": "ubuntu",
+      // we need to manually checkout the submodules,
+      // but VSCode may try to configure CMake before they are fully checked-out.
+      // workaround TBD
+      "postCreateCommand": "git submodule update --init"
+    }