Fix last session file for a prosession_dir with no trailing slash

[jdholtz]

When setting g:prosession_dir to a custom directory without a trailing slash, this causes the last session file to be saved as <directory>last_session.txt (notice no / between <directory> and last_session.txt.

This PR uses the fnamemodify to ensure a path separator is appended at the end (while still expanding ~).

With my configuration, I had:

init = function()
    vim.g.prosession_dir = vim.fn.stdpath("data") .. "/site/session"
end,

which caused the last_session file to be saved under ~/.local/share/nvim/site/sessionlast_session.txt.

[dhruvasagar]

@jdholtz Thanks for adding this, the PR looks good however the build is broken, i'll take a look and share my thoughts accordingly.

[jdholtz]

Sounds good. I can’t figure out what’s going wrong there. The tests pass locally for me and the failing test doesn’t seem to have to do with my modifications.

[dhruvasagar]

@jdholtz The test failure was a separate issue, thanks