Port over the clear command from nushell (#373)

* Port over the clear command from nushell

* cargo fmt

crates/nu-command/src/platform/clear.rs

@@ -0,0 +1,53 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Value,
+};
+use std::process::Command as CommandSys;
+
+#[derive(Clone)]
+pub struct Clear;
+
+impl Command for Clear {
+    fn name(&self) -> &str {
+        "clear"
+    }
+
+    fn usage(&self) -> &str {
+        "Clear the terminal."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("clear").category(Category::Platform)
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        if cfg!(windows) {
+            CommandSys::new("cmd")
+                .args(["/C", "cls"])
+                .status()
+                .expect("failed to execute process");
+        } else if cfg!(unix) {
+            CommandSys::new("/bin/sh")
+                .args(["-c", "clear"])
+                .status()
+                .expect("failed to execute process");
+        }
+
+        Ok(Value::Nothing { span: call.head }.into_pipeline_data())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Clear the terminal",
+            example: "clear",
+            result: None,
+        }]
+    }
+}