Add better exit command (#369)

crates/nu-protocol/src/signature.rs

@@ -42,6 +42,7 @@ pub enum Category {
     Filters,
     Formats,
     Math,
+    Shells,
     Strings,
     System,
     Viewers,
@@ -61,6 +62,7 @@ impl std::fmt::Display for Category {
             Category::Filters => "filters",
             Category::Formats => "formats",
             Category::Math => "math",
+            Category::Shells => "shells",
             Category::Strings => "strings",
             Category::System => "system",
             Category::Viewers => "viewers",