fix: return a String instead of a PathBuf in nvim_buf_get_name

[NyCodeGHG]

The buffer name is not always a path to a file,
it can also be e.g. a terminal buffer which's name starts with term:// and gets truncated to an empty string when used with PathBuf.

[noib3]

PathBuf::new doesn't truncate term://, and I've just added a test to confirm that Buffer::get_name returns the correct result in terminal buffers.

If you experience something else it's a bug, in which case please provide an example to test.

[NyCodeGHG]

oh i see, i didn't use display() on the PathBuf, i should have openend an issue first, thank you for your time :)