Migrate CI/CD from Azure Pipelines to GitHub Actions (#159)

- Add reusable workflows for each build target:
  - build-android.yml (macOS-14, JSC/V8, emulator tests)
  - build-ios.yml (Xcode version/simulator parameterized)
  - build-linux.yml (GCC/Clang, sanitizers)
  - build-macos.yml (Xcode, sanitizers)
  - build-uwp.yml (x64/arm64, Chakra/JSI/V8)
  - build-win32.yml (win32/x64, Chakra/JSI/V8, crash dumps)
- Add ci.yml orchestrating all 18 build jobs
- Remove azure-pipelines.yml and .github/jobs/

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: CedricGuillemet