bug#67463: 30.0.50; Eglot may manage js-json-mode buffers with wrong server

[Eli Zaretskii <eliz@gnu.org>]

> From: Pengji Zhang <kunhtkun@gmail.com>
> Date: Sun, 26 Nov 2023 16:15:54 -0500
> 
> It seems that buffers in `js-json-mode', the default mode for JSON
> files, could be wrongly managed by JavaScript language servers of Eglot,
> perhaps because:
> 
>   - `js-json-mode' is derived from `js-mode';
>   - Eglot uses `provided-mode-derived-p' to determine whether a buffer
>     should be managed by a server.
> 
> Here is a recipe to reproduce the problem I encountered:
> 
>   1. Install Node.js[0] and NPM[1]. (Sorry, but I could not find an
>      easier way to set up the environment. They may be installable with
>      your system's package manager.)
>   2. mkdir /tmp/repro && cd /tmp/repro
>   3. npm install typescript typescript-language-server (This will install
>      the language server locally in the directory.)
>   4. emacs -Q
>   5. C-x C-f main.js RET
>   6. C-u M-x eglot; enter "npx typescript-language-server --stdio"
>   7. C-x C-f package.json RET (This file is automatically created by
>      `npm install'.)
> 
> I expect that the 'package.json' buffer is not managed by Eglot, but it
> is actually managed by typescript-language-server, which does not really
> understand JSON and is reporting some invalid errors.
> 
> Thanks!
> 
> [0] https://nodejs.org/en
> [1] https://docs.npmjs.com/downloading-and-installing-node-js-and-npm

João, any suggestions?