Toggle list and grid display of Service Landing and Sub-landing page children

Introduces options for the display of service landing page child items following https://insidegovuk.blog.gov.uk/2025/01/31/rolling-out-simpler-ways-to-n... and discussion at https://github.com/localgovdrupal/localgov_services/issues/333 .

Via theme settings, site builders can optionally display the items as a list or in a number of columns of their choosing.

• Creates a theme setting for how many items in a row for service landing page child items
• Set default to 3 (so there is no change for current sites)
• Creates a theme setting for how many items in a row for service sub-landing page child items
• Set default to 2 (so there is no change for current sites)
• Creates hook_preprocess_field functions to pass the values of these settings to the corresponding field templates
• Amends the corresponding field templates to use these variables, with a fallback to what is currently in place so live sites will not break