fix(terminal): resuming doesn't work with command in fish (#37857)

Problem:  Resuming terminal process doesn't work with command in fish.
Solution: Send SIGCONT to the entire process group.

Use killpg() like what bash and zsh do on `fg`:
https://cgit.git.savannah.gnu.org/cgit/bash.git/tree/jobs.c?id=637f5c8696a6adc9b4519f1cd74aa78492266b7f#n3928
77045ef899/tree/Src/jobs.c (l2674)
77045ef899/tree/Src/signals.c (l538)

Install fish on CI to test this.

src/nvim/os/pty_proc_unix.c

@@ -241,7 +241,9 @@ void pty_proc_resize(PtyProc *ptyproc, uint16_t width, uint16_t height)
 
 void pty_proc_resume(PtyProc *ptyproc)
 {
-  kill(((Proc *)ptyproc)->pid, SIGCONT);
+  // Send SIGCONT to the entire process group, as some shells (e.g. fish) don't
+  // propagate SIGCONT to suspended child processes.
+  killpg(((Proc *)ptyproc)->pid, SIGCONT);
 }
 
 void pty_proc_close(PtyProc *ptyproc)