[mod] typification of SearXNG: add new result type File

This PR adds a new result type: File

    Python class: searx/result_types/file.py
    Jinja template: searx/templates/simple/result_templates/file.html
    CSS (less) client/simple/src/less/result_types/file.less

Class 'File' (singular) replaces template 'files.html' (plural).  The renaming
was carried out because there is only one file (singular) in a result. Not to be
confused with the category 'files' where in multiple results can exist.

As mentioned in issue [1], the class '.category-files' was removed from the CSS
and the stylesheet was adopted in result_types/file.less (there based on the
templates and no longer based on the category).

[1] https://github.com/searxng/searxng/issues/5198

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/templates/simple/result_templates/default.html

@@ -3,7 +3,7 @@
 {{ result_header(result, favicons, image_proxify) -}}
 {{- result_sub_header(result) -}}
 {% if result.iframe_src -%}
-<p class="altlink"><a class="btn-collapse collapsed media-loader disabled_if_nojs" data-target="#result-media-{{ index }}" data-btn-text-collapsed="{{ _('show media') }}" data-btn-text-not-collapsed="{{ _('hide media') }}">{{ icon('music-note') }} {{ _('show media') }}</a></p>
+<p class="altlink"><a class="btn-collapse collapsed media-loader disabled_if_nojs" data-target="#result-media-{{ index }}" data-btn-text-collapsed="{{ _('show media') }}" data-btn-text-not-collapsed="{{ _('hide media') }}">{{ icon('play') }} {{ _('show media') }}</a></p>
 {%- endif %}
 {%- if result.content %}
   <p class="content">