In nushell, the ENV_CONVERSIONS environment variable is used to
transform the defined variables from a string to a nushell value (PATH
to a list being one of the most common uses). This commit applies
user-defined conversions to direnv-loaded variables.
This fixes binary autocompletion not being triggered for newly added
paths and makes direnv work consistently with nushell
This enables nushell integration by default for direnv, similar to
bash/zsh/fish. The slightly verbose way of setting this is to ensure
that peoples' existing nushell configuration isn't overwritten, only
appended to, as would be the case if we just used the integration
example from the nushell docs:
https://www.nushell.sh/cookbook/direnv.htmlCloses#3520
