Implementing sparse attributes

[WebReflection]

In way too many occasions people assumed, or asked me about, the ability to have multiple holes per attribute, as in:

<div attr="${'a'}b${'c'}"/>

While I am still convinced having sparse attributes gives a false sense of partial attribute update, the DX seems to win over both logic and performance.

This MR is an attempt to bring partial attributes updates without penalizing performance too much, so that both hyperHTML and lighterhtml can benefit from this feature.

[WebReflection]

to whom it might concern, please feel free to thumb up or down this feature, thanks 👋