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-plugin-engine/src/declaration.rs

@@ -1,6 +1,6 @@
 use nu_engine::{command_prelude::*, get_eval_expression};
 use nu_plugin_protocol::{CallInfo, EvaluatedCall};
-use nu_protocol::{PluginIdentity, PluginSignature};
+use nu_protocol::{engine::CommandType, PluginIdentity, PluginSignature};
 use std::sync::Arc;
 
 use crate::{GetPlugin, PluginExecutionCommandContext, PluginSource};
@@ -116,8 +116,8 @@ impl Command for PluginDeclaration {
         )
     }
 
-    fn is_plugin(&self) -> bool {
-        true
+    fn command_type(&self) -> CommandType {
+        CommandType::Plugin
     }
 
     fn plugin_identity(&self) -> Option<&PluginIdentity> {