Feature request (config): Title/subtitle template

[FantomeBeignet]

I think it could be nice to give users more freedom to customize the title and subtitle. Setting the title (and possibly subtitle) config options with some kind of string interpolation syntax and predefined variables would allow for that.

Here is what I have in mind:

• Give the user access to some predefined variables such as command, program or cwd (for the current running command, program and current working directory). For example, running the command ping 1.1.1.1 in the home directory would set command="ping 1.1.1.1", program=ping, cwd=~.
• Allow the title (and possibly subtitle) config variables to be set with some kind of string interpolation syntax such as title = "Ghostty: ${cwd}".

This feature could also make for a better search experience using the tab overview (for example, having vim open from two different directories in two separate tabs currently doesn't allow the search to differentiate between the two tabs).

Slightly related to #3570 and #4095.

[pluiedev]

I like this proposal but I have reservations about introducing a string interpolation syntax. Would be a bit too much work on our end since we have to parse interpolations and manage to string them together — Zig's string interpolation happens entirely at compile time, so we cannot use its formatting mechanisms. Also the subtitle is only relevant on GTK (macOS windows can't have one).

Perhaps a better solution would be to allow for users to customize our shell integration into displaying any format that they want — then we don't have to interpret the format ourselves and only the shell does.