Introduce color module

[falkoschindler]

This PR is an alternative, simplified implementation of PR #2537. It introduces a color module and allows to use color constants like this:

from nicegui import color, ui

ui.icon('home', color=color.RED)
ui.icon('home', color=color.quasar.RED_5)
ui.icon('home', color=color.tailwind.RED_500)

Open tasks:

• pytest
• documentation
• color.RED can result in Quasar color
• uppercase/lowercase?

[falkoschindler]

I just noticed that

ui.icon('home', color=color.RED)

results in a Quasar color, because Quasar's "red" has the same name. We could avoid that by translating to the hex value:

RED = '#ff0000'