[css-gaps-1] Define gaps for grid lanes #14153

Context

• The normative definition of a gap per container type covers only multicol, flex, and grid containers. There is no grid
  lanes entry.
• The Applies to line of row-gap / column-gap
  (css-gaps-1 Row and Column Gutters section) likewise omits grid lanes
  containers.

Proposal

Grid axis (similar to grid / flex): The gutter is the space between adjacent
tracks. It runs the full length of the container along the stacking axis at a
fixed grid-axis position, exactly as column/row gaps do between grid tracks
(css-grid-1 §Gutters).

Stacking axis (defined relative to items, not cells): We define the
stacking-axis gutter relative to items:

1. A stacking-axis gap is the gutter immediately preceding an item along the stacking direction, for every item that is not the first in its track.
2. A stacking-axis gutter is positioned at its owning item's placed (running) position along the stacking axis. These gutters are independent per track and are not shared or aligned across tracks, unlike grid.
3. A spanner owns a single stacking-axis gutter immediately before it, spanning the grid-axis extent of all tracks it covers.