Multi level (multirow) header for table FAPI element

Relevant link https://www.w3.org/WAI/tutorials/tables/multi-level/

Worth looking at this example from We4Authors Cluster.
https://accessibilitycluster.com/main-results/table-creator

Would be associated with WCAG 1.3.1.

Adding a patch for D10 and attempting to fix cspell issue. Couldn't generate interdiff because of whitespace issue

Adding the patch again for correct version

Fixing ccf

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Fix "Warning: Undefined array key "content" in claro_preprocess_table() (line 1015 of /var/www/web/core/themes/claro/claro.theme)"

I have attached re-rolled patch for 10.1.x. please review

Using the examples module with form API
Added this to the form (copied from the test)

    $form['table'] = [
      '#type' => 'table',
      '#header' => [
        'row1' => [
          'bigcell' => [
            'data' => 'Big header',
            'rowspan' => 2,
          ],
          'longcell' => [
            'data' => 'Long header',
            'colspan' => 2,
          ],
          'Small header',
        ],
        'row2' => ['cell1', 'cell2', 'cell3'],
      ],
      '#header_multilevel' => TRUE,
      '#empty' => 'There are no lines here.',
    ];

Had to switch to olivero theme but it seemed to show correctly

But tried with a contrib theme and it broke completely so a change record will be needed and regular tables will have to be tested to make sure this isn't introducing a bug to existing sites.

For accessibility
Using the ANDI testing tool
Got 2 errors for the cells in pink Scope association needed at intersection of

Move changes from #71 to merge request

Has anybody tried this with a sticky table header?