CSS Grid: Support named grid lines and grid areas

[nicoburns]

What problem does this solve or what need does it fill?

This is an important part of the CSS Grid specification which is used by prominent websites such as Wikipedia.

What solution would you like?

• Add support for a list of named lines for the grid-template-{rows,columns} styles
• Add a new style property that defines grid-template-areas
• Support specifying a named line as a grid position. Using one of:
  • A seperate grid-area property
  • A merged grid-position property that accepts an enum of Area | Row+Column
  • Using the area-start and area-end grid lines to define the area.

What alternative(s) have you considered?

Make users of Taffy implement named grid lines and grid areas on top of Taffy themselves. I believe this is possible, but it would be nice to have the functionality built-in. We could consider feature flagging it to avoid the overhead where it is not needed.