gptel: Use json-mode to inspect query if available

[pabl0]

• gptel.el (gptel--inspect-query): Use json-mode when inspecting the query (dry run), if json-mode is available.

[karthink]

@pabl0 Thanks for the PR. I use fundamental-mode intentionally because both json-mode and the built-in js-json-mode suck when long lines are involved, slowing Emacs down to a crawl. And because long chunks of the buffer end up as a single line in the encoded JSON, this is always the case. (so-long mode does little to help here.)

In contrast, lisp-data-mode, which is used by the lisp-style query inspector, is much better at handling long lines.

If you wish to use json-mode you can advise the function in your configuration.