Feature: #98074 - Suggest CSS classes in link browser 

Description 

The CSS class field of the link browser for TCA fields of type link can now offer a list of predefined CSS classes. Editors pick a class from the list or still enter a custom one, similar to the existing choices of the target field.

The classes are configured via page TSconfig. Each entry consists of a value, which is the CSS class (or several space-separated classes), and an optional label, which can be a plain text or a localization reference. If no label is set, the value is shown.

Global configuration (applies to all link types):

TCEMAIN.linkHandler.properties.cssClass.items {
    10 {
        value = btn btn-primary
        label = my_extension.messages:linkClass.buttonPrimary
    }
    20 {
        value = link-arrow
        label = Arrow link
    }
}

Handler-specific configuration replaces the global list for that link type:

TCEMAIN.linkHandler.file.cssClass.items {
    10 {
        value = link-download
        label = Download
    }
}

The option complements cssClass.default, introduced with Feature: #107081 - Add options to predefine link target and class in link browser, which preselects a class for new links.

The CSS class field of the link browser in the rich text editor is not affected, its classes are still configured in the CKEditor configuration.

Impact 

Integrators can provide the CSS classes editors are supposed to use for links in TCA link fields, while editors can still enter any other class.