Introduce an html module with plain HTML elements

[falkoschindler]

Following up on #572 (reply in thread), this PR introduces an html module for creating plain HTML elements. They are basically shortcuts for ui.element(<tag>) and allow passing inner HTML as optional positional argument and arbitrary attributes as keyword arguments. Nesting, classes, styling and event subscriptions work like with regular NiceGUI elements. Here's a quick demo:

from nicegui import html

with html.p():
    html.a('Link', href='https://google.com').classes('text-red-500')
    html.hr()
    html.button('Button').on('click', lambda: print('Hello World'))
    html.img(src='https://picsum.photos/400/300', width='200')

Open tasks:

• Should we exclude "unusable" elements like html, head, and body? This might not be obvious in all cases.
• Add it to the documentation.
• Add pytests for some elements like button?

[rodja]

Great. I just added a small note and demo on how to use the props.