Since this is implemented as an accessibility feature on the OS level I don't think apps can influence what happens when using the mouse on iOS 13 beyond what Apple already offers as configuration options.

Thanks, I'll look into it.

Just syntax highlighting, it does not run code. 

I'll consider it for a future update. Go to line is already implemented by the way.

Textastic has a syntax definition for R.

Yes, I have seen this happen on my machine, too. I'm not sure why it happens yet. I'll need to investigate, but it's not easy to debug.

I just tried to reproduce this: if I enable "Smart Punctuation", I see the behavior you are seeing when using an external keyboard. If I disable "Smart Punctuation", I get regular quotation marks.

I've attached a screenshot of the setting that needs to be disabled: