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/build.rs

@@ -0,0 +1,3 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    nu_build::build()
+}