AK: Add String{,Utils}::to_snakecase()

This is an improved version of WrapperGenerator's snake_name(), which
seems like the kind of thing that could be useful elsewhere but would
end up getting duplicated - so let's add this to AK::String instead,
like to_{lowercase,uppercase}().

AK/String.cpp

@@ -389,6 +389,11 @@ String String::to_uppercase() const
     return m_impl->to_uppercase();
 }
 
+String String::to_snakecase() const
+{
+    return StringUtils::to_snakecase(*this);
+}
+
 bool operator<(const char* characters, const String& string)
 {
     if (!characters)