feat: external completions for commands/flags (#6295)

* wip

* wip

* cleanup

* error message

* cleanup

* cleanup

* fix clippy

* add test

* fix span

* cleanup

* cleanup

* cleanup

* fixed completion

* push char

* wip

* small fixes

* fix remove last span

* fmt

* cleanup

* fixes + more tests

* fix test

* only complete for commands

* also complete flags

* change decl_id to block_id

* use nu completion first

* fix test

* ignore test

* update config section

crates/nu-utils/src/sample_config/default_config.nu

@@ -231,8 +231,15 @@ let light_theme = {
     shape_nothing: light_cyan
 }
 
+# External completer example
+# let carapace_completer = {|spans| 
+#     carapace $spans.0 nushell $spans | from json
+# }
+
+
 # The default config record. This is where much of your global configuration is setup.
 let-env config = {
+  external_completer: $nothing # check 'carapace_completer' above to as example
   filesize_metric: false
   table_mode: rounded # basic, compact, compact_double, light, thin, with_love, rounded, reinforced, heavy, none, other
   use_ls_colors: true