fix: keep the newline char if file ends with that

[s12f]

I found if the input file ends with the '\n', tlauc will delete the newline char, I think tlauc should keep the file same as possible, files ends with the newline char are quite normal(as https://stackoverflow.com/questions/729692/why-should-text-files-end-with-a-newline said, the Posix standard defines a line should end with a newline char, and some editors(e.g. neovim) will automatically put a newline char in the end).

[ahelwer]

Thanks! Could you add some tests for this? In particular, I would like to see the behavior if empty input is provided.

[s12f]

@ahelwer added some tests, in the cases that input is empty or empty ends with a newline, the replace_symbols will panic in origin codes, so I add https://github.com/tlaplus-community/tlauc/pull/20/files#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R457-R459

[ahelwer]

Great changes, thanks!