MDL-76569 editor_tiny: Set table row header style

Configure Tiny to use markup of:

table > thead > tr > th

For all table headers.

lib/editor/tiny/amd/src/editor.js

@@ -242,6 +242,11 @@ const getStandardConfig = (target, tinyMCE, options, plugins) => {
         // https://www.tiny.cloud/docs/tinymce/6/statusbar-configuration-options/#branding
         branding: options.branding,
 
+        // Put th cells in a thead element.
+        // https://www.tiny.cloud/docs/tinymce/6/table-options/#table_header_type
+        // eslint-disable-next-line camelcase
+        table_header_type: 'sectionCells',
+
         setup: (editor) => {
             Options.register(editor, options);
         },