Introduce GHOSTTY_TERMINAL_OPT_PWD_CHANGED in libghostty-vt

[c0x0o]

Background

OSC 7 has already been proceeded by the libvt. But unlike what we did in title change, pwd change is never surfaced to the libvt API. This proposal to handle that

How large will the change be

About 120 lines change, with well-defined unit tests

Status

I've already have a patch for that, and test it for a while.

[jcollie]

!vouch