A refactor changed this block from a `set git_dir $PWD` to
`set git_dir $real_pwd`, but as `real_pwd` is a "static" variable and
not dynamically computed, this breaks within the `pushd` / `popd` flow.
Instead of using `$real_pwd`, we need to compute the *current* directory
by calling `__bobthefish_pwd` again.
Fixes#249.
* Add FNM as an alternative to NVM
* Made the __bobthefish_prompt_nvm generic for nvm and fnm
* Hide errors on node_manager current
* Check for color_nvm and set color_node based on that
