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-lsp/src/lib.rs

@@ -279,7 +279,7 @@ impl LanguageServer {
 
         match id {
             Id::Declaration(decl_id) => {
-                if let Some(block_id) = working_set.get_decl(decl_id).get_block_id() {
+                if let Some(block_id) = working_set.get_decl(decl_id).block_id() {
                     let block = working_set.get_block(block_id);
                     if let Some(span) = &block.span {
                         for cached_file in working_set.files() {