Detect if calc is needed and add it to style value

[tenphi]

We can detect syntax that requires calc usage. It will greatly simplify declarations. For example:

<nu-block width="100vw - 2br"></nu-block>

generated css:

nu-block[width="100vw - 2p"] {
  width: calc(100vw - (var(--nu-padding) * 2));
}