I thinki it's more easy to add it in you theme. No need use_custom_twig_extensions
1. Export template (and delete it)
2. unzip template
3. Add jquery.csv.js in scripts directory
4. Add <add>scripts/jquery.csv.js</add> in config.xml of your
template at script part
(sample for vanilla extended).
5. Zip your directory
6. Inport
With export/import and FTP access :
1. Add jquery.csv.js in scripts directory
2. Add <add>scripts/jquery.csv.js</add> in config.xml of your
template at script part
(sample for vanilla extended).
3. Reset options of theme