Similar to ctrl+c, ctrl+d can now be used to exit the program. To avoid accidental exit, ctrl+d must be pressed twice in relatively quick succession (same as ctrl+c).
Following common UX pattern, ctrl+d will be ignored when the input prompt is non-empty. This behavior is similar to how most shell (bash/zsh) behaves. To support this, I had to refactor so that text buffer is initialized outside of the InputPrompt component and instead do it on the main App component to allow input controller to have access to check the content of the text buffer.
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