On Windows, `builtin.help_tags` picker does not show any help tags.

To fix this, the following changes are needed:

1. `util.path_tail` checks unix separator `/` on Windows and leave the
original implementation intact on unix systems.

2. Line endings should be taken carefully on Windows. `vim.split` with
   only newline `\n` character as separator may result in unexpected
   crash when parsing large files. When splits on lines are needed, call
   it with `\r?\n`, or even set up a wrapper function in utils is more
   prefered.

Fixes nvim-telescope#3126