`code` command support custom directory when attaching to container

[Xieyt]

When running fm code <sitename>, VSCode currently opens with /workspace as the default directory. However, users should have the option to change this directory.

• [x] Implement the --work-dir flag, allowing users to define their preferred directory.
• [x] The default directory should be /workspace/frappe-bench.

[Xieyt]

When running fm code <sitename>, VSCode currently opens with /workspace as the default directory. However, users should have the option to change this directory.

* [x]  Implement the `--work-dir` flag, allowing users to define their preferred directory.

* [x]  The default directory should be `/workspace/frappe-bench`.

Implemented in #140.