feat: remove newspack-grid, newspack-grid-small, newspack-grid-sidebar

[thomasguillot]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This is a big one, so hopefully nothing’s broken.

This PR simplifies how columns are handled. I’ve removed the custom classes because it’s unlikely publishers will think to add them to their blocks, and they could easily be missed during migration. Instead, I’m targeting specific layouts directly, 25/50/25, 15/70/15, 33/66, and 66/33, and applying the grid to those.

The sidebar has been removed and replaced with the new grid. The same applies to newspack-grid-small, which felt redundant since the new grid covers those cases.

I’ve also removed the column patterns. Their main purpose was to include the newspack-grid class by default, but since that class is no longer used, keeping the patterns did not add much value.

Template parts have been checked to ensure everything still behaves as expected.

How to test the changes in this Pull Request:

1. On a page, add a bunch of columns, with newspack-grid (25/50/25, 15/70/15, 33/66, and 66/33)
2. On another page, add the same columns but without newspack-grid
3. Check and compare both pages
4. Switch to this branch
5. Refresh the page that didn't have the newspack-grid –– it should now look like the page you haven't refresh
6. Now refresh the other page and it should look exactly the same
7. Check various parts of your site if you notice anything broken
8. Check archive and search pages

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?