bug#69683: 28.2; eglot overrides imenu without fallback even when unsupported by server

[Eli Zaretskii <eliz@gnu.org>]

> From: Phil Hagelberg <phil@hagelb.org>
> Date: Sat, 09 Mar 2024 23:19:25 +0000
> 
> When invoking M-x imenu in a buffer with eglot enabled, it uses eglot's
> LSP-aware implementation of imenu. However, not all LSP servers handle
> that functionality. When connected to a server that does not offer this
> functionality, eglot signals an error:
> 
>   jsonrpc-error: "request id=11 failed:", (jsonrpc-error-code . -32601), (jsonrpc-error-message . "\"textDocument/documentSymbol\" is not in the request-handlers table"), (jsonrpc-error-data)
> 
> However, usually there is a perfectly good functional imenu
> implementation that the user wants to invoke; they just can't do it
> because eglot took over the imenu command.
> 
> Ideally eglot should only add its functionality to the imenu command
> when it's connected to a server that actually supports the endpoint
> needed to handle it.

Thanks.  Would you like to suggest a patch along these lines?

An alternative would be to catch this error and fall back on the imenu
implementation.  Not being an LSP expert, I'm unsure which approach is
better/cleaner.