Add .editorconfig file

Thanks to cweiske (https://github.com/adminerevo/adminerevo/pull/163).

.editorconfig

@@ -0,0 +1,19 @@
+# https://editorconfig.org/
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+
+[*.{php,css,js}]
+indent_style = tab
+
+[*.json]
+indent_style = space
+indent_size = 4
+
+[*.md]
+indent_style = space
+trim_trailing_whitespace = false
+max_line_length = 120