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
https://sourceforge.net/p/zsh/code/ci/77045ef899e53b9598bebc5a41db93a548a40ca6/tree/Src/jobs.c#l2674
https://sourceforge.net/p/zsh/code/ci/77045ef899e53b9598bebc5a41db93a548a40ca6/tree/Src/signals.c#l538

Install fish on CI to test this.

.github/scripts/install_deps.sh

@@ -31,7 +31,7 @@ if [[ $OS == Linux ]]; then
   fi
 
   if [[ -n $TEST ]]; then
-    sudo apt-get install -y locales-all cpanminus attr libattr1-dev gdb inotify-tools xdg-utils
+    sudo apt-get install -y locales-all cpanminus attr libattr1-dev fish gdb inotify-tools xdg-utils
 
     # Use default CC to avoid compilation problems when installing Python modules
     CC=cc python3 -m pip -q install --user --upgrade --break-system-packages pynvim
@@ -47,7 +47,7 @@ elif [[ $OS == Darwin ]]; then
   brew update --quiet
   brew install ninja
   if [[ -n $TEST ]]; then
-    brew install cpanminus fswatch
+    brew install cpanminus fish fswatch
 
     npm install -g neovim
     npm link neovim