[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>

.editorconfig

@@ -14,6 +14,14 @@ charset = utf-8
 # code formatter accepts length of 120, but editor should prefer 80
 max_line_length = 80
 
+[{*.sh,manage}]
+indent_style = space
+indent_size = 4
+
+# shfmt options
+shell_variant  = bash
+switch_case_indent = true
+
 [*.html]
 # in the jinja templates we use indent size of 2 and we do not use tabs
 indent_size = 2