fix: include NUL characters on calls to getLine

[axionbuster]

Fixes #3546 ("null characters break readFile/getLine").

This draft pull request showcases an implementation of getLine that would not truncate on the first occurrence of the NUL character (\0) and properly seek the newline character (assuming text mode) in case users expect getLine to preserve NUL characters in text input.

However, I was concerned that some parts of the Lean runtime might expect NUL-terminated strings. I am new to the codebase, so I'm not sure.

The repeated calls to ungetc may negatively affect performance. I'm not sure about this approach, either. I'm open to discussions.