This commit refactors the script execution logic across multiple muxers (zellij, tmux, and zellij) to simplify and standardize how startup and shutdown scripts are handled. The key changes include:

- **Unified Script Handling:**  Startup and shutdown scripts are now combined with other commands into a single list and executed together. This eliminates the separate `run_script` calls.
- **Command Combination:** The logic to include a script as a single command if it exists is now consistent across muxers.
- **Removed `run_script`:** The `run_script` method is removed as its functionality is now integrated into the `run_commands` method.

- Modifies script path construction by utilizing `std::env::temp_dir()` to create the script path.
- This change removes the hardcoded `/tmp` directory and dynamically uses the system's temporary directory.
- Updates test assertions across tmux and zellij tests to reflect the change in path construction by using `contains()` instead of strict equality.
- Improves portability and flexibility by avoiding a fixed temporary directory.

 - Adds `startup_script` to the configuration for custom startup commands.
 - Adds `shutdown_script` to the configuration for custom shutdown commands.
 - Adds `shell` config option for specifying a custom shell.
 - Adds `env` configuration section to optionally set environment variables.
 - Introduces a `script` option to pane definitions to run a custom script.
 - Improves example configuration snippets in both README.md and docs/content/docs/getting-started/basics.md to reflect new config options

- Extracts the `default_path` function into a new `common` module.
- Moves the implementation of `default_path` to `src/common/config/model/common.rs`.
- Updates the usage of the `default_path` function in `src/common/config/model/pane.rs` and `src/common/config/model/session.rs` to import from the new `common` module.
- Removes the unused `default_path` function from `src/common/config/util.rs`.

- Removes unnecessary `then` calls in `TmuxClient::stop_session` and `ZellijClient::stop_session`.
- Simplifies session stopping in `Tmux`, `Zellij` to improve readability.
- Updates `Tmux` and `Zellij` to use the new, refactored method.
- Consistently removes unnecessary references in shell command execution.

 - Removes the `env` block from the `.laito.yaml` file.
 - Removes the `startup_script` block from the `.laito.yaml` file.
 - Simplifies the configuration file structure by removing unnecessary values.