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-command/src/filesystem/open.rs

@@ -172,7 +172,7 @@ impl Command for Open {
                     match converter {
                         Some((converter_id, ext)) => {
                             let decl = engine_state.get_decl(converter_id);
-                            let command_output = if let Some(block_id) = decl.get_block_id() {
+                            let command_output = if let Some(block_id) = decl.block_id() {
                                 let block = engine_state.get_block(block_id);
                                 eval_block(engine_state, stack, block, stream)
                             } else {

crates/nu-command/src/filesystem/save.rs

@@ -393,7 +393,7 @@ fn convert_to_extension(
 ) -> Result<PipelineData, ShellError> {
     if let Some(decl_id) = engine_state.find_decl(format!("to {extension}").as_bytes(), &[]) {
         let decl = engine_state.get_decl(decl_id);
-        if let Some(block_id) = decl.get_block_id() {
+        if let Some(block_id) = decl.block_id() {
             let block = engine_state.get_block(block_id);
             let eval_block = get_eval_block(engine_state);
             eval_block(engine_state, stack, block, input)