Add filter facebook_for_woocommerce_fb_product_description on get_fb_description

[brezocordero]

Changes proposed in this Pull Request:

Closes #2756

Adds filter facebook_for_woocommerce_fb_product_description to allow filtering the return of get_fb_description

• [x] Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical. -> I have fixed the linting errors on the modified function only

Screenshots:

Detailed test instructions:

With a site connected to Facebook:

1. Add the filter to modify the product description, for example:

   add_filter( 'facebook_for_woocommerce_fb_product_description', function( $description ) {
   return $description . ' filtered';
   });

2. sync the products
3. View your catalog on Facebook and check the description is filtered

Additional details:

Changelog entry

Add - Filter facebook_for_woocommerce_fb_product_description