Implement native color picker support

Is your feature request about something that is currently impossible or hard to do? Please describe the problem.
I think it would be nice if MacVim supports native color picking. macOS provides a system color picker (NSColorPanel), and we could hook into it to allow Vim scripts to get the color picked by the user and update texts and whatnot. This feature is very useful for web development, but even things like developing Vim color schemes would benefit from this.

Describe the solution you'd like
We can provide provide an API that looks something like :call pick_color(callback, <options>). This will bring up a color panel that lets you pick the color. Once you are done, it will call the callback with the provided color, and the script can do whatever it wants.

<options> can specify a continuous mode (it's a feature of NSColorPanel) so that every time the user clicks on the color it will immediately send the color to the callback (without needing to close the panel) so it's much better for fast iteration. For example, the script can be hooked up so that every time it gets a new color it remotely commands Safari to reload the tab to preview the updated color you just picked.

Other options include: the current color, whether to force an output color space for the picked color (e.g. forcing the picked color to conform to sRGB), whether to use alpha or not.

Describe alternatives you've considered
There is already a plugin that does this (https://github.com/KabbAmine/vCoolor.vim) but it uses osascript to invoke AppleScript to activate a color picker. This way of invoking it doesn't provide a lot of customizable options (e.g. alpha vs no alpha), and it doesn't let you pick the colors in continuous mode. It's better to have native support.

Additional context
Ideally this can be made into a native Vim feature (e.g. Gtk also provides a color picker) so it can be used cross-platform. I would imagine the API needs some room for flexibility though (e.g. the Gtk one doesn't have continuous mode, and it appears it doesn't even let you pick a different color space like sRGB vs P3).

I'm not sure if this will work in terminal Vim. Probably not. You need an AppKit application in order to invoke this.

Also see VSCode's version of this: https://code.visualstudio.com/Docs/languages/css#_syntax-coloring-color-preview (it's not as powerful because it's just a built-in support for color picking in certain situations, but it's not an extensible API). Note that the other portion of previewing colors in place in Vim can be done in Vim 9 using virtual text (see https://www.reddit.com/r/vim/comments/wm08fl/simple_vim_9_virtual_text_example_for_hex_colors/)

未指定 repository 文件或测试；首先定位 MacVim 的 AppKit 集成以及现有的 Vim-script API 入口点。根据对 callback、options、continuous mode、color-space 和 alpha 的拟议要求，了解 NSColorPanel 的行为。完成的标准是 native API 能够使用所请求的 options 返回选定的颜色，同时处理 MacVim/AppKit 和 terminal 的限制。