Add imgui module package

[Sunrisepeak]

Summary

• add independent imgui package descriptor for imgui 0.0.1
• document imgui module usage and dependency chain in README
• add a smoke test for user-facing imports: imgui.core and imgui.backend.glfw_opengl3
• update CI to validate with released mcpp 0.0.45

Validation

• mcpp --version -> mcpp 0.0.45
• MCPP= timeout 1800 bash tests/smoke_imgui_module.sh