.nvmrc should be searched by traversing the directory tree up

[PegasisForever]

fish-nvm should include parent directories when searching .nvmrc.

For example:

$ echo 14 > .nvmrc
$ cd child
$ node -v
   # still 16

Somewhere related: jorgebucaran/nvm.fish#45

[FabioAntunes]

I won't be picking this up in the following months, maybe after summer. Feel free to open a PR if you need this sooner.