Add history and standard Ctrl & Alt commands to the REPL

[ageron]

This PR activates the REPL history and adds several Ctrl & Alt commands.

History

• The history was implemented, but not used. Now every command is appended to the history.
• When running the same command several times in a row, only the first command is appended to the history.
• When moving up the history then back all the way down, we get back the current line.
• After replaying command N, pressing down will select command N+1 (similar to Julia's REPL).
• Supports multiline commands (one line at a time).

Ctrl & Alt commands

• Ctrl-A: move cursor to start of line
• Ctrl-E: move cursor to end of line
• Ctrl-K: delete line to end
• Ctrl-U: delete line to start
• Ctrl-W: delete word backward
• Ctrl-Y: yank (i.e., paste whatever was deleted)
• Ctrl-B: move cursor left
• Ctrl-F: move cursor right
• Ctrl-H: delete character before
• Alt-B: move word left
• Alt-F: move word right
• Alt-D: delete word forward

This makes editing very pleasant on Mac and Linux as the terminals are often configured to generate these commands when typing common keystrokes (e.g., Cmd-Backspace generates Ctrl-U).

Note that I edited Unix.zig to set new_termios.lflag.ISIG and new_termios.lflag.IEXTEN to false to ensure that the REPL receives key combos like Ctrl-Y (or else this suspends the process).

Implemented mostly by antigravity. I reviewed all the code.