feat: spawn the executables directly if possible (#3974)

* feat: spawn the executables directly if possible

This pull request changes nu-command so that it spawns the process directly if:
- They are a `.exe` on Windows
- They are not a `.sh` or `.bash` on not windows.

Benefits:
- As I explained in [this comment](https://github.com/nushell/nushell/issues/3898#issuecomment-894000812), this is another step towards making Nushell a standalone shell, that doesn't need to shell out unless it is running a script for a particular shell (cmd, sh, ps1, etc.).
- Fixes the bug with multiline strings
- Better performance due to direct spawning.

For example, this script shows ~20 ms less latency.
After:
```nu
C:\> benchmark { node -e 'console.log("sssss")' }
───┬──────────────────
 # │    real time
───┼──────────────────
 0 │ 63ms 921us 600ns
───┴──────────────────
```
Before
```nu
C:\> benchmark { node -e 'console.log("sssss")' }
───┬──────────────────
 # │    real time
───┼──────────────────
 0 │ 79ms 136us 800ns
───┴──────────────────
```

Fixes #3898

* fix: make which dependency optional

Also fixes clippy warnings

* refactor: refactor spawn_exe, spawn_cmd, spawn_sh, and spawn_any

* fix: use which feature instead of which-support

* fix: use which_in to use the cwd of nu

* fix: use case insensitive comparison of the extensions

Sometimes the case of the extension is uppercased by the "which_in" function

Also use unix instead of not windows. Some os might not have sh support

crates/nu-command/src/classified/external.rs

@@ -4,7 +4,10 @@ use nu_engine::{MaybeTextCodec, StringOrBinary};
 use nu_test_support::NATIVE_PATH_ENV_VAR;
 use parking_lot::Mutex;
 
+#[allow(unused)]
+use std::env;
 use std::io::Write;
+use std::path::PathBuf;
 use std::process::{Command, Stdio};
 use std::sync::mpsc;
 use std::{borrow::Cow, io::BufReader};
@@ -17,6 +20,9 @@ use nu_protocol::hir::{ExternalCommand, ExternalRedirection};
 use nu_protocol::{Primitive, ShellTypeName, UntaggedValue, Value};
 use nu_source::Tag;
 
+#[cfg(feature = "which")]
+use which::which_in;
+
 pub(crate) fn run_external_command(
     command: ExternalCommand,
     context: &mut EvaluationContext,
@@ -141,19 +147,18 @@ fn run_with_stdin(
     )
 }
 
-fn spawn(
+/// Spawn a direct exe
-    command: &ExternalCommand,
+#[allow(unused)]
-    path: &str,
+fn spawn_exe(full_path: PathBuf, args: &[String]) -> Command {
-    args: &[String],
+    let mut process = Command::new(full_path);
-    input: InputStream,
+    for arg in args {
-    external_redirection: ExternalRedirection,
+        process.arg(&arg);
-    scope: &Scope,
+    }
-) -> Result<InputStream, ShellError> {
+    process
-    let command = command.clone();
+}
 
-    let mut process = {
+/// Spawn a cmd command with `cmd /c args...`
-        #[cfg(windows)]
+fn spawn_cmd_command(command: &ExternalCommand, args: &[String]) -> Command {
-        {
     let mut process = Command::new("cmd");
     process.arg("/c");
     process.arg(&command.name);
@@ -165,17 +170,67 @@ fn spawn(
         process.arg(&arg);
     }
     process
-        }
+}
 
-        #[cfg(not(windows))]
+/// Spawn a sh command with `sh -c args...`
-        {
+fn spawn_sh_command(command: &ExternalCommand, args: &[String]) -> Command {
     let cmd_with_args = vec![command.name.clone(), args.join(" ")].join(" ");
     let mut process = Command::new("sh");
     process.arg("-c").arg(cmd_with_args);
     process
-        }
+}
-    };
 
+/// a function to spawn any external command
+#[allow(unused)] // for minimal builds cwd is unused
+fn spawn_any(command: &ExternalCommand, args: &[String], cwd: &str) -> Command {
+    // resolve the executable name if it is spawnable directly
+    #[cfg(feature = "which")]
+    // TODO add more available paths to `env::var_os("PATH")`?
+    if let Result::Ok(full_path) = which_in(&command.name, env::var_os("PATH"), cwd) {
+        if let Some(extension) = full_path.extension() {
+            #[cfg(windows)]
+            if extension.eq_ignore_ascii_case("exe") {
+                // if exe spawn it directly
+                return spawn_exe(full_path, args);
+            } else {
+                // TODO implement special care for various executable types such as .bat, .ps1, .cmd, etc
+                // https://github.com/mklement0/Native/blob/e0e0b8785cad39a73053e35084d1f60d87fbac58/Native.psm1#L749
+                // otherwise shell out to cmd
+                return spawn_cmd_command(command, args);
+            }
+            #[cfg(not(windows))]
+            if !["sh", "bash"]
+                .iter()
+                .any(|ext| extension.eq_ignore_ascii_case(ext))
+            {
+                // if exe spawn it directly
+                return spawn_exe(full_path, args);
+            } else {
+                // otherwise shell out to sh
+                return spawn_sh_command(command, args);
+            }
+        }
+    }
+    // in all the other cases shell out
+    if cfg!(windows) {
+        spawn_cmd_command(command, args)
+    } else {
+        // TODO what happens if that os doesn't support spawning sh?
+        spawn_sh_command(command, args)
+    }
+}
+
+fn spawn(
+    command: &ExternalCommand,
+    path: &str,
+    args: &[String],
+    input: InputStream,
+    external_redirection: ExternalRedirection,
+    scope: &Scope,
+) -> Result<InputStream, ShellError> {
+    let command = command.clone();
+
+    let mut process = spawn_any(&command, args, path);
     process.current_dir(path);
     trace!(target: "nu::run::external", "cwd = {:?}", &path);