[mod] simple theme: change stylelint configuration

* disable declaration-empty-line-before
  https://stylelint.io/user-guide/rules/list/declaration-empty-line-before/
  this change allows to mix CSS declarations and LESS mixins without empty lines:

  #something {
    display: flex;
    .ltr-left(60rem); // no mandatory empty line before this one
  }

* disable no-invalid-position-at-import-rule
  https://stylelint.io/user-guide/rules/list/no-invalid-position-at-import-rule/

  this change allows to declare some mixins and then import another .less file:
  for example:

  .ltr-left(@offset) {
    left: @offset;
  }
  @import "style.less";

searx/static/themes/simple/.stylelintrc.json

@@ -1,3 +1,7 @@
 {
-  "extends": "stylelint-config-standard"
+  "extends": "stylelint-config-standard",
+  "rules": {
+    "declaration-empty-line-before": null,
+    "no-invalid-position-at-import-rule": null
+  }
 }