feat: 79 add option to add datetime to filename

[joncrangle]

Responds to issue: #79 .

The user can provide an optional date_fmt string in the strftime syntax interpreted by wezterm.strftime(). If this opt is provided, the current local datetime will be appended to the saved filename for the state and visible in the fuzzy picker when restoring or deleting it.

--- e.g.
     action = wezterm.action_callback(function(win, pane)
       resurrect.save_state(resurrect.workspace_state.get_workspace_state(), nil, "%Y-%m-%d %H:%M:%S")
     end),
---
     action = resurrect.window_state.save_window_action(nil, "%Y-%m-%d %H:%M:%S"),
---
     action = resurrect.tab_state.save_tab_action(nil, "%Y-%m-%d %H:%M:%S"),

I added the optional opt_name param to the save_window_action and save_tab_action functions to make them consistent with the resurrect.save_state args.

[MLFlexer]

I really like that it is done using Lua! Much better than using external processes like ls -l

minor comments about the args and code style

[MLFlexer]

I have been thinking about this PR and also reviewed #82 which tries to tackle the same issue.

I think that I will go with #82 because this PR will add a file each time the state is saved, i.e. when saving the state, it will add a new version of the state with the name <state_name> <date>. This can fill the state directories with a lot of versions of the same state, however I would think a user would only care about the newest version of the state. Thus making the older versions redundant.

Please correct me if I am wrong, but because of this reason, I think I will close this PR and merge #82 instead.
But again, thank you for taking the time trying to fix the issue! 😄