Surprising symlink resolution for std path add (#13258)

# Description
The standard library's `path add` function has some surprising side
effects that I attempt to address in this PR:

1. Paths added, if they are symbolic links, should not be resolved to
their targets. Currently, resolution happens.

   Imagine the following:

   ```nu
# Some time earlier, perhaps even not by the user, a symlink is created
   mkdir real-dir
   ln -s real-dir link-dir

   # Then, step to now, with link-dir that we want in our PATHS variable
   use std
   path add link-dir
   ```

In the current implementation of `path add`, it is _not_ `link-dir` that
will be added, as has been stated in the command. It is instead
`real-dir`. This is surprising. Users have the agency to do this
resolution if they wish with `path expand` (sans a `--no-symlink` flag):
for example, `path add (link-dir | path expand)`

In particular, when I was trying to set up
[fnm](https://github.com/Schniz/fnm), a Node.js version manager, I was
bitten by this fact when `fnm` told me that an expected path had not
been added to the PATHS variable. It was looking for the non-resolved
link. The user in [this
comment](https://github.com/Schniz/fnm/issues/463#issuecomment-1710050737)
was likely affected by this too.

Shells, such as nushell, can handle path symlinks just fine. Binary
lookup is unaffected. Let resolution be opt-in.

Lastly, there is some convention already in place for **not** resolving
path symlinks in the [default $env.ENV_CONVERSIONS
table](57452337ff/crates/nu-utils/src/sample_config/default_env.nu (L65)).
   
2. All existing paths in the path variable should be left untouched.
Currently, they are `path expand`-ed (including symbolic link
resolution).

   Path add should mean just that: prepend/append this path.

Instead, it currently means that, _plus mutate all other paths in the
variable_.

Again, users have the agency to do this with something like `$env.PATH =
$env.PATH | split row (char esep) | path expand`.

3. Minorly, I update documentation on running tests in
`crates/nu-std/CONTRIBUTING.md`. The offered command to run the standard
library test suite was no longer functional. Thanks to @weirdan in [this
Discord
conversation](https://discord.com/channels/601130461678272522/614593951969574961/1256029201119576147)
for the context.

# User-Facing Changes

(Written from the perspective of release notes)

- The standard library's `path add` function no longer resolves symlinks
in either the newly added paths, nor the other paths already in the
variable.

# Tests + Formatting

A test for the changes working correctly has been added to
`crates/nu-std/tests/test_std.nu` under the test named
`path_add_expand`.

You can quickly verify this new test and the existing `path add` test
with the following command:

```nu
cargo run -- -c 'use crates/nu-std/testing.nu; NU_LOG_LEVEL=INFO testing run-tests --path crates/nu-std --test path_add'
```

All commands suggested in the issue template have been run and complete
without error.

# After Submitting
I'll add a release note to [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged.

crates/nu-std/CONTRIBUTING.md

@@ -204,7 +204,7 @@ More design guidelines:
 ### Useful Commands
 - Run all unit tests for the standard library:
   ```nushell
-  cargo run -- -c 'use std testing; testing run-tests --path crates/nu-std'
+  cargo run -- -c 'use crates/nu-std/testing.nu; testing run-tests --path crates/nu-std'
   ```
   > **Note**  
   > this uses the debug version of NU interpreter from the same repo, which is
@@ -216,7 +216,7 @@ More design guidelines:
 - Run all tests for a specific test module, e.g,
   `crates/nu-std/tests/test_foo.nu`
   ```nushell
-  cargo run -- -c 'use std testing; testing run-tests --path crates/nu-std --module test_foo'
+  cargo run -- -c 'use crates/nu-std/testing.nu; testing run-tests --path crates/nu-std --module test_foo'
   ```
 - Run a custom command with additional logging (assuming you have instrumented
   the command with `log <level>`, as we recommend.)

crates/nu-std/std/mod.nu

@@ -66,7 +66,7 @@ export def --env "path add" [
             "record" => { $p | get --ignore-errors $nu.os-info.name },
         }
 
-        $p | path expand
+        $p | path expand --no-symlink
     }
 
     if null in $paths or ($paths | is-empty) {
@@ -80,7 +80,6 @@ export def --env "path add" [
         $env
             | get $path_name
             | split row (char esep)
-            | path expand
             | if $append { append $paths } else { prepend $paths }
     )}
 

crates/nu-std/tests/test_std.nu

@@ -38,12 +38,38 @@ def path_add [] {
         std path add $target_paths
         assert equal (get_path) ([($target_paths | get $nu.os-info.name)] | path expand)
 
-        load-env {$path_name: [$"/foo(char esep)/bar"]}
+        load-env {$path_name: [$"(["/foo", "/bar"] | path expand | str join (char esep))"]}
         std path add "~/foo"
         assert equal (get_path) (["~/foo", "/foo", "/bar"] | path expand)
     }
 }
 
+#[test]
+def path_add_expand [] {
+    use std assert
+
+    # random paths to avoid collision, especially if left dangling on failure
+    let real_dir = $nu.temp-path | path join $"real-dir-(random chars)"
+    let link_dir = $nu.temp-path | path join $"link-dir-(random chars)"
+    mkdir $real_dir
+    let path_name = if $nu.os-info.family == 'windows' {
+        mklink /D $link_dir $real_dir
+        "Path"
+    } else {
+        ln -s $real_dir $link_dir | ignore
+        "PATH"
+    }
+
+    with-env {$path_name: []} {
+        def get_path [] { $env | get $path_name }
+
+        std path add $link_dir
+        assert equal (get_path) ([$link_dir])
+    }
+
+    rm $real_dir $link_dir
+}
+
 #[test]
 def banner [] {
     std assert ((std banner | lines | length) == 15)