Update direnv.md - use ENV_CONVERSIONS

[Jasha10]

This PR updates the nushell cookbook's direnv integration recommendation so that $env.PATH will be stored as a nushell list rather than as a colon-separated string.

Before this change, my $env.PATH looks like this:

> $env.PATH
/Users/jasha/bin:/Users/jasha/.cargo/bin:/Users/jasha/.local/bin:/Users/jasha/.nvm/versions/node/v18.19.0/bin:/Users/jasha/.pyenv/shims:/opt/homebrew/bin:/opt/homebrew/sbin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Applications/iTerm.app/Contents/Resources/utilities

After the change, `$env.PATH looks like this:

> $env.PATH
╭────┬───────────────────────────────────────────────────────────────────────────────────╮
│  0 │ /Users/jasha/bin                                                                  │
│  1 │ /Users/jasha/.cargo/bin                                                           │
│  2 │ /Users/jasha/.local/bin                                                           │
│  3 │ /Users/jasha/.nvm/versions/node/v18.19.0/bin                                      │
│  4 │ /Users/jasha/.pyenv/shims                                                         │
│  5 │ /opt/homebrew/bin                                                                 │
│  6 │ /opt/homebrew/sbin                                                                │
│  7 │ /bin                                                                              │
│  8 │ /sbin                                                                             │
│  9 │ /usr/bin                                                                          │
│ 10 │ /usr/sbin                                                                         │
│ 11 │ /usr/local/bin                                                                    │
│ 12 │ /System/Cryptexes/App/usr/bin                                                     │
│ 13 │ /var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin         │
│ 14 │ /var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin               │
│ 15 │ /var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin │
│ 16 │ /Applications/iTerm.app/Contents/Resources/utilities                              │
╰────┴───────────────────────────────────────────────────────────────────────────────────╯

I have verified that the direnv integration still works as expected, with $env.PATH being modified when I cd into or out of a direnv-managed directory.

[Jasha10]

Question for reviewers: should we add defensive logic for the case where the user has not set up their $env.ENV_CONVERSIONS table?

E.g.

            if 'ENV_CONVERSIONS' in ($env | columns) and 'PATH' in ($env.ENV_CONVERSIONS | columns) {
              $env.PATH = do $env.ENV_CONVERSIONS.PATH.from_string $env.PATH
            }

[fdncred]

should we add defensive logic ...

yes. with the current release, I think it's possible to not have ENV_CONVERSIONS at all. the next release may change that though.

[Jasha10]

@fdncred updated in commit f0edb31

[fdncred]

Thanks