Insomnia supports various text editor key maps,under Preferences > General > Font > Text Editor Key Map.
To select a text editor key map, go to the font section of the preferences general tab.
The key maps currently available are Vim, Emacs, and Sublime.
By default, press-and-hold on macOS shows special characters.
This is not particularly useful when using the Vim key map, because navigation is restricted to one move at a time. In order to enable key-repeating, execute the following in your terminal and restart Insomnia:
defaults write com.insomnia.app ApplePressAndHoldEnabled -bool false