start command in nushell (#1727)

* mvp for start command

* modified the signature of the start command

* parse filenames

* working model for macos is done

* refactored to read from pipes

* start command works well on macos; manual testing reveals need of --args flag support

* implemented start error; color printing of warning and errors

* ran clippy and fixed warnings

* fix a clippy lint that was caught in pipeline

* fix dead code clippy lint for windows

* add cfg annotation to import

crates/nu_plugin_start/src/nu/mod.rs

@@ -0,0 +1,25 @@
+use nu_errors::ShellError;
+use nu_plugin::Plugin;
+use nu_protocol::{CallInfo, Signature, SyntaxShape, Value};
+
+use crate::start::Start;
+
+impl Plugin for Start {
+    fn config(&mut self) -> Result<Signature, ShellError> {
+        Ok(Signature::build("start")
+            .desc("Opens each file/directory/URL using the default application")
+            .rest(SyntaxShape::String, "files/urls/directories to open")
+            .named(
+                "application",
+                SyntaxShape::String,
+                "Specifies the application used for opening the files/directories/urls",
+                Some('a'),
+            ))
+    }
+    fn sink(&mut self, call_info: CallInfo, input: Vec<Value>) {
+        self.parse(call_info, input);
+        if let Err(e) = self.exec() {
+            println!("{}", e);
+        }
+    }
+}