Define keywords (#13213)

# Description
Some commands in `nu-cmd-lang` are not classified as keywords even
though they should be.

# User-Facing Changes
In the output of `which`, `scope commands`, and `help commands`, some
commands will now have a `type` of `keyword` instead of `built-in`.

crates/nu-cmd-lang/src/core_commands/break_.rs

@@ -1,4 +1,5 @@
 use nu_engine::command_prelude::*;
+use nu_protocol::engine::CommandType;
 
 #[derive(Clone)]
 pub struct Break;
@@ -18,6 +19,15 @@ impl Command for Break {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn run(
         &self,
         _engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/continue_.rs

@@ -1,4 +1,5 @@
 use nu_engine::command_prelude::*;
+use nu_protocol::engine::CommandType;
 
 #[derive(Clone)]
 pub struct Continue;
@@ -18,6 +19,14 @@ impl Command for Continue {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
     fn run(
         &self,
         _engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -2,7 +2,7 @@ use nu_engine::{
     command_prelude::*, get_eval_block, get_eval_expression, get_eval_expression_with_input,
 };
 use nu_protocol::{
-    engine::StateWorkingSet,
+    engine::{CommandType, StateWorkingSet},
     eval_const::{eval_const_subexpression, eval_constant, eval_constant_with_input},
 };
 
@@ -41,6 +41,15 @@ impl Command for If {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn is_const(&self) -> bool {
         true
     }

crates/nu-cmd-lang/src/core_commands/loop_.rs

@@ -1,4 +1,5 @@
 use nu_engine::{command_prelude::*, get_eval_block};
+use nu_protocol::engine::CommandType;
 
 #[derive(Clone)]
 pub struct Loop;
@@ -20,6 +21,15 @@ impl Command for Loop {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/match_.rs

@@ -1,7 +1,7 @@
 use nu_engine::{
     command_prelude::*, get_eval_block, get_eval_expression, get_eval_expression_with_input,
 };
-use nu_protocol::engine::Matcher;
+use nu_protocol::engine::{CommandType, Matcher};
 
 #[derive(Clone)]
 pub struct Match;
@@ -27,6 +27,15 @@ impl Command for Match {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/scope/command.rs

@@ -1,5 +1,4 @@
 use nu_engine::{command_prelude::*, get_full_help};
-use nu_protocol::engine::CommandType;
 
 #[derive(Clone)]
 pub struct Scope;
@@ -20,10 +19,6 @@ impl Command for Scope {
         "Commands for getting info about what is in scope."
     }
 
-    fn command_type(&self) -> CommandType {
-        CommandType::Keyword
-    }
-
     fn run(
         &self,
         engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/try_.rs

@@ -1,5 +1,5 @@
 use nu_engine::{command_prelude::*, get_eval_block, EvalBlockFn};
-use nu_protocol::engine::Closure;
+use nu_protocol::engine::{Closure, CommandType};
 
 #[derive(Clone)]
 pub struct Try;
@@ -31,6 +31,15 @@ impl Command for Try {
             .category(Category::Core)
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,

crates/nu-cmd-lang/src/core_commands/while_.rs

@@ -1,4 +1,5 @@
 use nu_engine::{command_prelude::*, get_eval_block, get_eval_expression};
+use nu_protocol::engine::CommandType;
 
 #[derive(Clone)]
 pub struct While;
@@ -29,6 +30,15 @@ impl Command for While {
         vec!["loop"]
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn command_type(&self) -> CommandType {
+        CommandType::Keyword
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,