LibJS: Make Value() default-construct the undefined value

The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().

The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.

Tests/LibJS/test-value-js.cpp

@@ -86,7 +86,7 @@ TEST_CASE(non_canon_nans)
         EXPECT(!val.is_integral_number());     \
         EXPECT(!val.is_finite_number());       \
         EXPECT(!val.is_infinity());            \
-        EXPECT(!val.is_empty());               \
+        EXPECT(!val.is_special_empty_value()); \
         EXPECT(!val.is_nullish());             \
     }