more precise parsing of cmd through the information in nu.scope.commands (#522)

- refactor `after` to `after`, `before`
- `gp` support `back-to-prev` for merging scenes
- `ka`, `ke`, `kep`, `ked`, `kes` supports labels selector

Co-authored-by: agent <agent@nuc>

modules/after/after.nu

@@ -2,8 +2,20 @@ def "nu-complete ps" [] {
     ps -l | each {|x| { value: $"($x.pid)", description: $x.command } }
 }
 
-# after { do something ... } <pid>
-export def main [action, pid: string@"nu-complete ps"] {
+# after <pid> {|| do something ... }
+export def main [
+    pid: string@"nu-complete ps"
+    action
+] {
+    do -i { tail --pid $pid -f /dev/null }
+    do $action
+}
+
+# before {|| do something ... } <pid>
+export def before [
+    action
+    pid: string@"nu-complete ps"
+] {
     do -i { tail --pid $pid -f /dev/null }
     do $action
 }