This is in Textastic 3.0.

The new highlighter is now in Textastic 3.0 and supports TextMate syntax definitions and themes.

I've added a combined undo/redo button instead (segmented control) in Textastic 3.0.

You can add your own TextMate-compatible bundles and themes in Textastic 3.0.

New syntax highlighter with Monokai theme and the symbol list:

It's currently in beta and I'm soon going to send it to Apple for review.

In Textastic 3.0 you'll be able to use custom syntax definitions. There's a TextMate bundle available that you'll be able to use.

Since you write that you would buy the app if it supported this: Did you already try it out with Textastic or do you assume that it doesn't work?

Do you have any example code that doesn't currently work in Textastic? That would make it much easier for me to support this.