Add devcontainer configuration for zha-device-handlers project

[cemizm]

Proposed change

Add a VS Code devcontainer configuration for zha-device-handlers to make local development and testing easier and more reproducible:

• Add a .devcontainer/Dockerfile based on mcr.microsoft.com/devcontainers/python:1-3.12 with common build dependencies for Python packages.
• Add .devcontainer/devcontainer.json to configure the workspace, recommended extensions and a postCreateCommand that runs script/setup to create the virtualenv and install dev dependencies.
• Add .devcontainer/README.md documenting what the devcontainer does and how to use it with VS Code / Dev Containers.

This change is purely for development workflow convenience and does not affect any runtime code, quirks, or released packages.
Motivation: make onboarding easier for contributors, have a reproducible dev environment, and keep the local development environment of contributors (including mine) clean and less cluttered.

Additional information

• No functional changes to ZHA quirks or device handling.
• No impact on Home Assistant users; this only affects contributors who opt into using the devcontainer / GitHub Codespaces.
• The intention of this PR is to improve the contributor experience and provide a clean, isolated environment for development without polluting the host system with additional tooling and dependencies.
• I’m not sure if adding a .devcontainer setup is desired in this repository; if this is not welcome or does not fit the maintainers' preferences, I’m happy to adjust the PR or close it again.

Device diagnostics

Not applicable – this PR does not add or modify any device quirks and only introduces development tooling (.devcontainer).

Checklist

• The changes are tested and work correctly
• pre-commit checks pass / the code has been formatted using Black
• Tests have been added to verify that the new code works
• Device diagnostics data has been attached

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (bf73aa9) to head (0962abb).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #4509   +/-   ##
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  Coverage   92.24%   92.24%           
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  Files         369      369           
  Lines       12094    12094           
=======================================
  Hits        11156    11156           
  Misses        938      938           

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