AK: Add invert_case() and invert_case(StringView)

In the given String, invert_case() swaps lowercase characters with
uppercase ones and vice versa.

AK/String.cpp

@@ -388,6 +388,11 @@ String String::to_titlecase() const
     return StringUtils::to_titlecase(*this);
 }
 
+String String::invert_case() const
+{
+    return StringUtils::invert_case(*this);
+}
+
 bool operator<(char const* characters, String const& string)
 {
     return string.view() > characters;