Feature: for newly created prompt key-up deletes whole text

## Problem
On Mac, when you typed a very long multiline prompt, you usually expect that key-up only switch line for cursor

Instead of this the actual behavior is as follows:
- codewhale forgets what was actually typed by user
- codewhale shows the previous command

If the typed prompt was big enough, it is rather painful to understand that you need to remember it all and retype again

## Proposed solution

For if:

- there is a not empty prompt or comand in a composer
- composer contains several lines of text (and not just one)
- cursor is not on the topmost line
then instead of switching to prevoius entered message cursor just jumps 1 line up

## Use case
In every big enought prompt it will be useful
The same is actual also for key-down

## Alternatives considered
Current behavior deletes un-entered message

## Impact
Rather painful message deletion

Start at the composer’s key-up and key-down handling, focusing on how multiline prompts and the cursor’s line position are determined. Reproduce the issue with a long multiline prompt on Mac, then verify that navigation moves one line within the prompt without replacing it with the previous command, while topmost and single-line cases retain their existing behavior.

Written by the indexing model from the issue text.