Multiple bug fixes and support for Welcart extension plug-ins

【Contents of fixes】
■Added support for WCEX SKU Select extension plugin.
■Added support for the WCEX DLSeller extension plugin.
■Fixed the bug that the number of items displayed in the “Designed Article List” widget is not reflected.
■Fixed a bug that the display of ads on both “left” and “right” in the “Ads 2” section of the article details page was broken.
■Fixed an issue where the catchphrase on the fixed page would not display properly in IE11.
■Fixed an issue where the link button on the top page banner content would be displayed even when viewed on a smartphone.
■Fixed an issue where the overlay transparency value could not be set to 0 in the header image settings for fixed pages.
■Fixed the comment function.
■Added wording to theme options.

The following are the files that have been modified for this update.
【Corrected files】
admin/inc/basic.php
functions/head.php
functions/page_cf2.php
functions/short_code.php
language/ja.mo
language/ja.po
template-parts/page-header.php
wc_templates/wc_item_single.php
wc_templates/cart/wc_completion_page.php
wc_templates/cart/wc_confirm_page.php
wc_templates/cart/wc_customer_page.php
wc_templates/cart/wc_delivery_page.php
widget/styled_post_list_tab.php
functions.php
style.css

【Added files】
js/comment.js
template-parts/related-recommend-item.php
wc_templates/wc_item_single_data.php
wc_templates/wc_item_single_service.php
wc_templates/wc_sku_select.php
wc_templates/wc_sku_select_data.php
wc_templates/wc_sku_select_service.php
wcex_sku_select.css