Add <nu-props/> decorator to declare custom properties

[tenphi]

With nu-props use can customize built-in properties and declare your own:

• [indent] --nu-indent (default: .5rem) – Base indent (padding|gap).
• [border-width] --nu-border-width (default: 1px) – Base border width.
• [border-radius] --nu-border-radius (default: .5rem) – Base border radius.
• [animation-time] --nu-animation-time (default: 0.08s) – Base animation time.

Example of declaration:

<nu-props
  indent=".5"
  border-radius=".5"
  border-width="1px"
  animation-time="0.08s"
  your-custom-property="(100vw - 2rem)"></nu-props>