Allow defining named grid lines on the grid element
This change adds parsing, style resolution and getComputedStyle
support for named grid lines at the grid element level (ie extends
our <track-list> support). Per the specification, we allow multiple
grid lines with the same name.
To fully support resolving the grid lines to a position on our
grid, we need to add the parsing at the grid item's level (which
means extending our <grid-line> support). This will be done in a