feat(cli, flatpak): implemented kill action for pwsp-cli.

use it instead of pkill in the flatpak wrapper

src/types/commands.rs

@@ -18,6 +18,8 @@ pub trait Executable {
 
 pub struct PingCommand {}
 
+pub struct KillCommand {}
+
 pub struct PauseCommand {
     pub id: Option<u32>,
 }
@@ -93,6 +95,13 @@ impl Executable for PingCommand {
     }
 }
 
+#[async_trait]
+impl Executable for KillCommand {
+    async fn execute(&self) -> Response {
+        Response::new(true, "killed")
+    }
+}
+
 #[async_trait]
 impl Executable for PauseCommand {
     async fn execute(&self) -> Response {