LibWeb: Replace "+" in value with a space while decoding search params

Tests/LibWeb/Text/expected/URL/search-params-with-plus-in-value.txt

@@ -0,0 +1,2 @@
+key 'key'
+value 'value1 value2'

Tests/LibWeb/Text/input/URL/search-params-with-plus-in-value.html

@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const params = new URLSearchParams('key=value1+value2');
+        for (const [key, value] of params) {
+            println(`key '${key}'`);
+            println(`value '${value}'`);
+        }
+    });
+</script>