Unified setDark API. Happens to take care of meta

[evnchn]

Motivation

There are 2 problems this PR want to address:

1. Tell browser when dark mode is enabled #5471, where select browsers and Dark Reader extension do not cope well with NiceGUI's dark mode because we did not use <meta> tags to inform them
2. ui.dark_mode is hard-coded to use Quasar.Dark.set which gets in the way of using other UI frameworks than Quasar (Let configure UI framework other than Quasar #4859)

Real motivation: to experiment, since I have doubts over "demo is good enough instead of API addition" #5471 (comment)

Implementation

• Quasar.Dark.set will never be called directly: It will be wrapped to take true/false/undefined, then set as darkSetter
• darkSetter can be another UI library's dark mode function, if you need that.
• setDark calls the darkSetter while also handles the <meta> tags for you.
• To prevent FOUC, the <meta> tags are also populated in HTML.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests are not necessary? (not sure...)
• Documentation is not necessary? (not sure...)
• Maybe let's use more unique id names to avoid collision with user's code?