Use CommandType in more places (#12832)

# Description
Kind of a vague title, but this PR does two main things:
1. Rather than overriding functions like `Command::is_parser_keyword`,
this PR instead changes commands to override `Command::command_type`.
The `CommandType` returned by `Command::command_type` is then used to
automatically determine whether `Command::is_parser_keyword` and the
other `is_{type}` functions should return true. These changes allow us
to remove the `CommandType::Other` case and should also guarantee than
only one of the `is_{type}` functions on `Command` will return true.
2. Uses the new, reworked `Command::command_type` function in the `scope
commands` and `which` commands.


# User-Facing Changes
- Breaking change for `scope commands`: multiple columns (`is_builtin`,
`is_keyword`, `is_plugin`, etc.) have been merged into the `type`
column.
- Breaking change: the `which` command can now report `plugin` or
`keyword` instead of `built-in` in the `type` column. It may also now
report `external` instead of `custom` in the `type` column for known
`extern`s.

crates/nu-engine/src/eval.rs

@@ -36,10 +36,10 @@ pub fn eval_call<D: DebugContext>(
             &decl.examples(),
             engine_state,
             caller_stack,
-            decl.is_parser_keyword(),
+            decl.is_keyword(),
         );
         Ok(Value::string(full_help, call.head).into_pipeline_data())
-    } else if let Some(block_id) = decl.get_block_id() {
+    } else if let Some(block_id) = decl.block_id() {
         let block = engine_state.get_block(block_id);
 
         let mut callee_stack = caller_stack.gather_captures(engine_state, &block.captures);

crates/nu-engine/src/scope.rs

@@ -111,13 +111,8 @@ impl<'e, 's> ScopeData<'e, 's> {
                     "signatures" => self.collect_signatures(&signature, span),
                     "usage" => Value::string(decl.usage(), span),
                     "examples" => Value::list(examples, span),
-                    // we can only be a is_builtin or is_custom, not both
-                    "is_builtin" => Value::bool(!decl.is_custom_command(), span),
+                    "type" => Value::string(decl.command_type().to_string(), span),
                     "is_sub" => Value::bool(decl.is_sub(), span),
-                    "is_plugin" => Value::bool(decl.is_plugin(), span),
-                    "is_custom" => Value::bool(decl.is_custom_command(), span),
-                    "is_keyword" => Value::bool(decl.is_parser_keyword(), span),
-                    "is_extern" => Value::bool(decl.is_known_external(), span),
                     "creates_scope" => Value::bool(signature.creates_scope, span),
                     "extra_usage" => Value::string(decl.extra_usage(), span),
                     "search_terms" => Value::string(decl.search_terms().join(", "), span),