[imgui] create with v1.92.5, add webgpu-binding (dawn)

[luncliff]

📝 Pull Request

This PR adds a new port for imgui (v1.92.5) and introduces a webgpu-binding feature that depends on dawn.

Summary

• Port: imgui
• Version: v1.92.5
• Feature added: webgpu-binding (depends on dawn)
• Build system: CMake

What I tested locally

• vcpkg install --overlay-ports "./ports" imgui — succeeded for x64-windows.
• vcpkg install --overlay-ports "./ports" "imgui[webgpu-binding]:wasm32-emscripten" — succeeded (Emscripten/dawn built and produced JS glue in share/dawn/src).

Both installs used workspace-local buildtrees/, packages/, and installed/ roots.

CI status

• Several CI checks passed (Linux, macOS, Emscripten, many Windows checks). Some overlay/Windows hosted jobs are unknown or show failure in current runs — please inspect CI logs for platform-specific issues.

Notes and recommendations

• webgpu-binding adds dawn which pulls multiple dependencies; expect longer build times on first run. Binary caching is recommended for CI.
• The Emscripten build used a community triplet (wasm32-emscripten) from C:\vcpkg\triplets\community\wasm32-emscripten.cmake. Consider adding a stable local triplets/wasm32-emscripten.cmake to the repo for reproducible CI.
• Please ensure the versions/ baseline is updated if you intend to publish this port.

Checklist (local verification)

• Port installs successfully with overlay (--overlay-ports) on x64-windows.
• Port installs successfully with webgpu-binding on wasm32-emscripten.
• No unexpected files in port directory (please review).
• License file present in share/imgui/ (installed — please verify contents).
• Port formatted (vcpkg format-manifest --all).
• Version baseline updated (if publishing).

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For reviewers: check share/imgui/*.cmake, exported targets, and feature behavior on target platforms. If you want, I can run /review-port checks next and add suggested edits to the portfiles.