feat: add nushell support

Author: LittleJianCH

src/cli/self_update.rs

@@ -384,6 +384,7 @@ the corresponding `env` file under {cargo_home}.
 This is usually done by running one of the following (note the leading DOT):
     . "{cargo_home}/env"            # For sh/bash/zsh/ash/dash/pdksh
     source "{cargo_home}/env.fish"  # For fish
+    source "{cargo_home}/env.nu"    # For nushell
 "#
     };
 }

src/cli/self_update/env.nu

@@ -0,0 +1,3 @@
+if ("{cargo_bin}" not-in ($env.Path | split row (char esep))) {
+  $env.Path = ($env.Path | prepend "{cargo_bin}")
+}

src/cli/self_update/shell.rs

@@ -77,6 +77,7 @@ fn enumerate_shells() -> Vec<Shell> {
         Box::new(Bash),
         Box::new(Zsh),
         Box::new(Fish),
+        Box::new(Nu),
     ]
 }
 
@@ -255,6 +256,46 @@ impl UnixShell for Fish {
     }
 }
 
+struct Nu;
+
+impl UnixShell for Nu {
+    fn does_exist(&self, process: &Process) -> bool {
+        // nu has to either be the shell or be callable for nu setup.
+        matches!(process.var("SHELL"), Ok(sh) if sh.contains("nu"))
+            || utils::find_cmd(&["nu"], process).is_some()
+    }
+
+    fn rcfiles(&self, process: &Process) -> Vec<PathBuf> {
+        process.var("XDG_CONFIG_HOME").ok().and(
+            process.home_dir()
+        ).iter().flat_map(|path| {
+            let mut p0 = path.clone();
+            let mut p1 = path.clone();
+            p0.push("nushell/env.nu");
+            p1.push("nushell/config.nu");
+            [p0, p1]
+        }).collect()
+    }
+
+    fn update_rcs(&self, process: &Process) -> Vec<PathBuf> {
+        self.rcfiles(process)
+            .into_iter()
+            .filter(|rc| rc.is_file())
+            .collect()
+    }
+
+    fn env_script(&self) -> ShellScript {
+        ShellScript {
+            name: "env.nu",
+            content: include_str!("env.nu")
+        }
+    }
+
+    fn source_string(&self, process: &Process) -> Result<String> {
+        Ok(format!(r#"source "{}/env.nu""#, cargo_home_str(process)?))
+    }
+}
+
 pub(crate) fn legacy_paths(process: &Process) -> impl Iterator<Item = PathBuf> + '_ {
     let zprofiles = Zsh::zdotdir(process)
         .into_iter()