[simple theme] activate - SearXNG JavaScript Style Guide

Add the 'SearXNG JavaScript Style Guide' as one quality gate in the build chain
of the simple theme::

    make themes.simple
    make themes.simple.test

In the github CI the `themes.all` target enforce a `themes.simple`.

BTW: Remove 'jshint' left overs from 0ee316f3d

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

manage

@@ -714,6 +714,17 @@ themes.simple() {
     dump_return $?
 }
 
+themes.simple.test() {
+    build_msg TEST "theme: simple"
+    if ! nvm.min_node "${NODE_MINIMUM_VERSION}"; then
+        info_msg "install Node.js by NVM"
+        nvm.nodejs
+    fi
+    npm --prefix searx/static/themes/simple install
+    npm --prefix searx/static/themes/simple run test
+    dump_return $?
+}
+
 PYLINT_FILES=()
 while IFS= read -r line; do
    PYLINT_FILES+=("$line")