fix(nushell): adapt the completer to the 0.116 contract (#4934)

* fix(nushell): adapt the completer to the 0.116 contract

Nushell 0.116 runs an external completer on a background worker
without the terminal unless the registered closure reaches a
command carrying the @interactive attribute. Interactivity is
seen through the closure to the command it calls, so wrap the
completion logic in an @interactive entry point and register a
closure that ends with the call to it.

Read the named `place` input instead of the deprecated
positional `spans`; $place.command carries the same token list.

An @interactive completer cannot decline with null (null reads
as a dismissed picker), so request the fallback explicitly when
the `**` trigger is absent, keeping plain file completion
working. The previous external completer is still called when
the trigger is absent.

The script now requires Nushell 0.116+, since @interactive is a
parse error on older versions.

Fixes #4932

* fix(nushell): retry the previous completer with the legacy spans input

The fallback to the previously configured external completer always
handed it the new `place` input. A completer that still declares the
legacy `spans` parameter errors on a record, so the fallback silently
produced no completions for pre-0.116 completers (e.g. older Carapace
installs).

Try `place` first, then retry with the token list, which is what the
old positional bridge handed such completers.

* docs(nushell): note the 0.116 requirement and load order

Requires Nushell 0.116.1+, where a completion can take the terminal
through the @interactive attribute, and the wrapper captures the
external completer present at load time.

README.md

@@ -841,6 +841,13 @@ Fuzzy completion in Nushell works via the
 [external completer](https://www.nushell.sh/cookbook/external_completers.html)
 mechanism. There are some differences compared to bash and zsh:
 
+- Nushell 0.116.1 or newer is required. The completion runs on the line-editor
+  thread so fzf can take the terminal; a previously configured external
+  completer (e.g. Carapace) is therefore invoked inline when the `**` trigger
+  is absent.
+- The script wraps whichever external completer is configured when it loads,
+  so load it after any other external completer setup. The autoload setup
+  described above runs after `config.nu`, so it already does this.
 - On Nushell >= 0.103.0, the external completer is no longer called for
   built-in commands (e.g. `cd`, `ls`). Fuzzy completion with `**<TAB>` only
   works for external commands.