[dev/tooling]: add basic devcontainer setup w/ node, pnpm, go, nix & recommended tools

Add a development container for contributors and document how to use it.

- Add .devcontainer/devcontainer.json
- Add .devcontainer/Dockerfile
- Update docs/CONTRIBUTING.md with Dev Container section

- devcontainer.json parses successfully and opens in vscode without issues
- all build.sh variants, recommended tools & lint/typecheck run without issues

Closes #499

.devcontainer/Dockerfile

@@ -0,0 +1,7 @@
+FROM mcr.microsoft.com/devcontainers/base:bookworm
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends build-essential libnss3-tools mkcert pkg-config && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*