Port over the clear command from nushell (#373)

* Port over the clear command from nushell

* cargo fmt

crates/nu-protocol/src/signature.rs

@@ -42,6 +42,7 @@ pub enum Category {
     Filters,
     Formats,
     Math,
+    Platform,
     Shells,
     Strings,
     System,
@@ -62,6 +63,7 @@ impl std::fmt::Display for Category {
             Category::Filters => "filters",
             Category::Formats => "formats",
             Category::Math => "math",
+            Category::Platform => "platform",
             Category::Shells => "shells",
             Category::Strings => "strings",
             Category::System => "system",