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-parser/src/known_external.rs

@@ -1,5 +1,8 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::ast::{Argument, Expr, Expression};
+use nu_protocol::{
+    ast::{Argument, Expr, Expression},
+    engine::CommandType,
+};
 
 #[derive(Clone)]
 pub struct KnownExternal {
@@ -22,12 +25,8 @@ impl Command for KnownExternal {
         &self.usage
     }
 
-    fn is_known_external(&self) -> bool {
-        true
-    }
-
-    fn is_builtin(&self) -> bool {
-        false
+    fn command_type(&self) -> CommandType {
+        CommandType::External
     }
 
     fn run(

crates/nu-parser/src/parse_keywords.rs

@@ -1020,7 +1020,7 @@ pub fn parse_alias(
                 } => {
                     let cmd = working_set.get_decl(rhs_call.decl_id);
 
-                    if cmd.is_parser_keyword()
+                    if cmd.is_keyword()
                         && !ALIASABLE_PARSER_KEYWORDS.contains(&cmd.name().as_bytes())
                     {
                         working_set.error(ParseError::CantAliasKeyword(

crates/nu-parser/src/parser.rs

@@ -5966,7 +5966,7 @@ pub fn discover_captures_in_expr(
         Expr::Bool(_) => {}
         Expr::Call(call) => {
             let decl = working_set.get_decl(call.decl_id);
-            if let Some(block_id) = decl.get_block_id() {
+            if let Some(block_id) = decl.block_id() {
                 match seen_blocks.get(&block_id) {
                     Some(capture_list) => {
                         // Push captures onto the outer closure that aren't created by that outer closure