feat(nix): nix and back again

I tried Nix, but it had too many downsides so I removed it.

1. Didn't like that all files are immutable and simple config changes
need a complete rebuild.
2. Setting up a new Mac didn't work as smoothly as promised. Not worth
the effort.
3. It sucked a lot to always have to type in the password twice on each
darwin-rebuild
4. It solves problems I never had.

.functions/fkill.sh

@@ -0,0 +1,25 @@
+# fkill - kill process
+
+fkill() {
+    local pid
+    local signal="${1:-9}"
+    local pattern="$2"
+
+    if ! [ "$signal" -eq "$signal" ] 2>/dev/null; then
+        echo "Invalid signal: $signal"
+        return 1
+    fi
+
+    if [ -n "$pattern" ]; then
+        pid=$(ps -ef | sed 1d | grep "$pattern" | fzf -m --header='[kill:process]' --preview 'echo {}' --preview-window down:3:wrap | awk '{print $2}')
+    else
+        pid=$(ps -ef | sed 1d | fzf -m --header='[kill:process]' --preview 'echo {}' --preview-window down:3:wrap | awk '{print $2}')
+    fi
+
+    if [ -n "$pid" ]; then
+        echo "Killing processes with PID: $pid"
+        echo "$pid" | xargs kill "-$signal"
+    else
+        echo "No process selected."
+    fi
+}