[enh] CI - add shell script formatter

Implement rules and functions to format shell scripts:

    $ make format.shell

or alternatively to format all source code:

    $ make format

The formatter `shfmt` reads the rules from the editorconfig[1]

  If any EditorConfig files are found, they will be used to apply formatting
  options.  If any parser or printer flags are given to the tool, no
  EditorConfig files will be used.

[1] https://github.com/patrickvane/shfmt?tab=readme-ov-file#description

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

utils/lib_sxng_test.sh

@@ -6,8 +6,9 @@ test.help(){
 test.:
   yamllint  : lint YAML files (YAMLLINT_FILES)
   pylint    : lint ./searx, ./searxng_extra and ./tests
-  pyright   : static type check of python sources (.dev or .ci)
   black     : check black code format
+  shfmt     : check shfmt code format
+  shfmt     : check Shell script code format
   unit      : run unit tests
   coverage  : run unit tests with coverage
   robot     : run robot test
@@ -105,6 +106,12 @@ test.black() {
     dump_return $?
 }
 
+test.shfmt() {
+    build_msg TEST "[shfmt] ${SHFMT_SCRIPTS[*]}"
+    go.tool shfmt --list --diff "${SHFMT_SCRIPTS[@]}"
+    dump_return $?
+}
+
 test.unit() {
     build_msg TEST 'tests/unit'
     # shellcheck disable=SC2086