refactor(file_selector): optimize handling of selected files

Sorting function in FileSelector:get_filepaths() is not very efficient
since it has to hit the filesystem each time it is called to determine
if it is dealing with a directory or not.

Optimize performance by figuring this data in get_project_filepaths()
and returning a list of structures containing paths and directory flags,
and use this data in get_filepaths(). Do the absolute->relative
conversion once.

Also use vim.fn.isdirectory() instead of vim.loop.fs_stat() everywhere
in the file.