Add a CSS variable for the label in the elements nuxeo-document-suggestion and nuxeo-directory-suggestion

People need to customize the CSS of the "<label>" tag for the elements nuxeo-document-suggestion and nuxeo-directory-suggestion. Currently it is not possible because no CSS variable is exposed in these elements for this tag.

So this ticket is a request to add it, as it is already done for some layout with

  <template>
    <style>
...

      label {
        @apply --nuxeo-label;
      }