Merge branch '5522-editorconfig' into 'develop'

Propose a basic editorconfig, #5522

Closes #5522

See merge request ispconfig/ispconfig3!1738

.editorconfig

+; top-most EditorConfig file
+root = true
+
+; Unix-style newlines
+[*]
+charset = utf-8
+end_of_line = LF
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = tab
+
+[*.{htm,html}]
+indent_style = space
+indent_size = 4