css: compare visibility keyword values case-insensitively

CSS keywords are ASCII case-insensitive, but StyleManager matched
display:none / visibility:hidden|collapse / opacity:0 / pointer-events:none
with exact String.eql, so `style="DISPLAY: NONE"` or a `.x { Visibility:
Hidden }` rule left the element visible to checkVisibility,
getComputedStyle, the semantic tree and interactiveElements.

Compare at the check sites via a new String.eqlSliceIgnoreCase rather than
lowercasing declared values, which would corrupt content, url() and custom
property values that el.style must reflect verbatim.

src/string.zig

@@ -224,6 +224,10 @@ pub const String = extern struct {
         };
     }
 
+    pub fn eqlSliceIgnoreCase(a: String, b: []const u8) bool {
+        return std.ascii.eqlIgnoreCase(a.str(), b);
+    }
+
     const EqualOrDeleted = union(enum) {
         deleted,
         equal: bool,