Implement a history buffer in the REPL class that stores previously entered commands and allows cycling through them using the up and down arrow keys. The `_history` list and `_historyIndex` track the current position, with `previousHistory()` and `nextHistory()` methods restoring lines from the buffer or clearing the input when reaching the end. The `executeInput()` method now appends the current line to history before resetting, and the Ctrl-D exit condition is fixed to check `_line.isEmpty` instead of `_line == ""`.
Bob Nystrom
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