add mouseInside window tracking

[treeform]

Summary

• Add window.mouseInside on macOS, Windows, Linux, and Emscripten so apps can tell when the cursor has left the window.
• Add examples/mouse_inside.nim: red while the mouse is over the window, black when it is not.
• Compile that example in Github Actions on every OS, plus Emscripten on Ubuntu.

Test plan

• Confirm CI compiles examples/mouse_inside.nim on ubuntu, macos, and windows.
• Confirm CI compiles it with -d:emscripten on Ubuntu.
• Run nim c -r examples/mouse_inside.nim locally: window is red with the mouse inside, black after the mouse leaves, and it does not stay red after leave.

Made with Cursor