Add public API to get the state for focus in/out reporting

[Martin Hostettler]

Currently the status of focus in/out reporting in only available internally in libvterm to suppress reporting in vterm_state_focus_in/vterm_state_focus_out.

It would be useful to be able to query this state for libvtermdriver (a program to do automated testing for terminal interaction).

[Martin Hostettler (textshell)]

I've implemented this in 2 variants now. I was not sure if this should be exposed as a simple getter `vterm_state_focus_reporting_enabled` or via a VTermProp. So i implemented both.

See attached patches.

[Paul "LeoNerd" Evans (leonerd)]

It seems neatest to do it as a termprop; that way applications can be informed when its value changes (in case they need to hook up extra event handlers, etc...)

I'll take the first one, though renamed `VTERM_PROP_FOCUSREPORT` since none of the other props (e.g. cursor-related ones) have additional underscores in parts of the names.

It also needs some tests, which I shall add.

[Paul "LeoNerd" Evans (leonerd)]

838: Paul "LeoNerd" Evans 2023-08-03 Implement a VTERM_PROP_FOCUSREPORT so applications can be told when focus reporting is enabled (LP1852009)

[Paul "LeoNerd" Evans (leonerd)]

Fixed in v0.3.3