Introduce ui.add_style for CSS, SASS and SCSS

[falkoschindler]

This PR introduces an add_style function for adding CSS, SASS and SCSS style definitions to the head HTML:

ui.add_style('''
    .red {
        color: red;
    }
''')
ui.label('This is red with CSS.').classes('red')

ui.add_style('''
    .green {
        background-color: lightgreen;
        .blue {
            color: blue;
        }
    }
''')
with ui.element().classes('green'):
    ui.label('This is blue on green with SCSS.').classes('blue')

ui.add_style('''
    .yellow
        background-color: yellow
        .purple
            color: purple
''', indented=True)
with ui.element().classes('yellow'):
    ui.label('This is purple on yellow with SASS.').classes('purple')

It would be a nice feature for NiceGUI 2.0. But technically it doesn't break anything and, thus, doesn't have to wait for a major release.

Still missing:

• tests
• documentation