Syntax for line-names in grid-template-rows/columns should be updated to use brackets

The csswg accepted a change to use ```[line-name]``` instead of ```(line-name)``` when giving names to lines between tracks. The polyfill and its examples are still built on the previous iteration which used the parentheses. That should be updated (and it should be straightforward to do so, though fixing the examples is really what will take the most time).

Discovered by @Aloso initially in issue #26