LibWeb: Fix parsing bug for SVG attributes

This doesn't seem to actually have fixed any bugs, as having
FillOpacity instead of StrokeOpacity in the call to parse_css_value
doesn't seem to have actually been causing bugs. But, I still think it's
worthwhile correcting.

The reason that it wasn't causing bugs is that having FillOpacity
instead of StrokeOpacity in the call to parse_css_value means that when
parsing the value is compared to the acceptable values for that property
(for example the value can only be a percentage, or a number, etc.). In
this case both FillOpacity and StrokeOpacity seem to accept the same
values.

Tests/LibWeb/Layout/input/svg/svg-different-types-of-opacity.html

@@ -0,0 +1,11 @@
+<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
+  <circle
+    cx="40"
+    cy="40"
+    r="25"
+    stroke-opacity="50%"
+    fill-opacity="20%"
+    stroke="green"
+    opacity="90%"
+    stroke-width="20" />
+</svg>