Visually separate scriptlet parameters in active line

This makes it easier to see how parameters are parsed internally,
in order to make it easier to distinguish commas as separator and
literal commas meant to be part of a parameter.

Related discussion:
uBlockOrigin/uBlock-discussions#865 (reply in thread)

src/css/codemirror.css

@@ -134,6 +134,10 @@
 .cm-theme-override .cm-s-default .cm-variable {
     color: var(--sf-variable-ink);
     }
+.cm-theme-override .cm-s-default .CodeMirror-activeline .cm-ext-js .cm-variable {
+    text-decoration: underline color-mix(in srgb, var(--sf-variable-ink) 30%, transparent) solid 3px;
+    text-decoration-skip-ink: none;
+    }
 .cm-theme-override .cm-s-default .cm-warning {
     background-color: var(--sf-warning-surface);
     text-decoration: underline var(--sf-warning-ink);