[Gap Decorations]: Build Main Gaps for Grid

This CL sets up the Main gaps for the MC Gap Geometry in grid. The row
direction is used as the primary one, so this change tracks all the
midpoint block offsets for all row gaps in a grid. Subsequent CLs will
involve:

* Building the Cross gaps
* Handling painting in the simple case using both main and cross gaps
* Adding spanners logic for grid
* Painting decorations with spanners

See: third_party/blink/renderer/core/layout/gap/README.md for more.

Bug: 436140061, 357648037
Change-Id: I775073a33d89bd133a19d3a0d0fbd6a91e2f08ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6854089
Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Sam Davis Omekara <samomekarajr@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1503530}