Crash on startup when no argument is given or the file doesn't exist #1

I want to report three problems that happen during startup:

1. If you run rusk (or cargo run) without arguments, the program panics at src/main.rs:69 (expect("Use: rusk ")).
2. If you run rusk missing.txt with a file that doesn't exist, the program panics at src/editor/buffer.rs:15 (read_to_string(file).unwrap()).
3. In both cases the error message is printed in a "staircase" pattern, and after the crash the terminal no longer responds correctly to input.

The causes are:

1. The program enables raw mode (enable_raw_mode()) and the alternate screen before reading the arguments. In raw mode \n doesn't move the cursor back to the start of the line, which is why the output looks like a staircase. Since the program exits with a panic, disable_raw_mode() is never called and the terminal stays in raw mode.
2. buffer::fetch assumes the file always exists and calls unwrap() on the result of read_to_string.

To fix it, we can read the argument before enabling raw mode. If it's missing, print the usage to stderr and exit with code 1 instead of panicking.
For the missing file, we can just create it and handle it with an empty buffer.
Finally, we can add a panic hook that disables raw mode.