OSC-99 query for support

[dnkl]

There's a discussion on https://web.libera.chat/?channels=#senpai (https://sr.ht/~delthas/senpai/), on how an application should choose which notification protocol to use.

Neither OSC-777, OSC-99, nor OSC-9 can be auto-detected in a meaningful way (I don't consider querying the terminal name and version meaningful).

Would you be open to adding a way to query for OSC-99 support?

For example:

Query:

OSC 99 ; ? ST

or, if you want to keep the parameter+payload format:

OSC 99 ; ? ; ST

Response:

OSC 99 ST

I'm hoping the protocol as a whole is simple enough that we don't need to include which features/parameters are supported.

[kovidgoyal]

Feel free to comment if you have concerns.

[dnkl]

Looks good to me. @delthas?

[kovidgoyal]

I also added urgency to the query response, since there might be
platforms that don't support it.

[delthas]

LGTM. Thanks for accepting and merging this so fast! :)

[kovidgoyal]

Apologies, I forgot about supporting multiplexers in the query/response, see c4baed7