docs: add devcontainer for examples

[hasnainkhatri87]

Summary

Add a Dev Container configuration for Moss contributors.

Changes

• Provide Python 3.12, Node.js 22, and Go 1.23 in Codespaces/dev containers.
• Install the root, Python example, JavaScript example, and Go example dependencies after container creation.
• Forward Moss credentials from the local environment without committing secrets.
• Include common VS Code language extensions and development ports.

Testing

• Parsed and structurally validated .devcontainer/devcontainer.json locally.
• git diff --check passed.
• npm run lint was attempted but is blocked on Windows by the existing ESLINT_USE_FLAT_CONFIG=true eslint . script syntax before ESLint starts.
• Full container startup could not be run because Docker is not installed in the development environment.

Closes #378

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

Adds a repository-level Dev Container configuration to make it easier for contributors to run Moss examples in Codespaces / VS Code Dev Containers with a preinstalled toolchain.

Changes:

• Introduces .devcontainer/devcontainer.json using a base Ubuntu image with Python 3.12, Node.js 22, and Go.
• Runs a postCreateCommand to install Python, Node, and Go example dependencies.
• Configures forwarded ports and editor extensions for common workflows.

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