I would see Lasso 9 in the list of supported languages. In LassoSoft website, you can find language reference. Thank you Alexander, great app!
you can add your own syntax definitions to Textastic using TextMate bundles.
There seems to be a TextMate bundle for Lasso at https://github.com/bfad/Lasso-TM-Bundle
Here's how to install it in Textastic:
Customer support service by UserEcho