feat: sync internal clipboard with system

[druxorey]

This pull request resolves #7, integrating the system clipboard, allowing yanked (copied) and deleted cell content to be available outside the application. The main changes include importing the github.com/atotto/clipboard package, serializing clipboard data to a tab-separated string, and writing it to the system clipboard during yank and delete operations.

System Clipboard Integration:

• Added github.com/atotto/clipboard as a dependency in go.mod to enable cross-platform clipboard access.
• Imported the clipboard package as sysclip in internal/sheets/clipboard.go.
• Implemented the String() method for the clipboard type to serialize clipboard contents as tab-separated values, with rows separated by newlines.
• Updated storeYankClipboard and storeDeleteClipboard methods to write the serialized clipboard data to the system clipboard after yanking or deleting cells.

[maaslalani]

Hey! Thank you so much for the contribution. I wouldn't feel comfortable erasing the contents of the clipboard automatically. We should add a separate keybinding to copy to the system clipboard ("y+) for example.

[druxorey]

Honestly, it seemed the most natural way to me; that's how I have it configured in my nvim config. But if you think this is better, that's perfect for me. I'll fix it in in a while and upload a new commit.

[druxorey]

@maaslalani I was implementing the y+ feature but I ran into a problem that I'm not sure how you'd prefer to solve. In normal mode, when I press the y key, It waits for another key to be pressed before deciding what to do, so adding this functionality is trivial. The problem arises with visual mode, because just like in Vim, pressing the y key doesn't wait for anything and simply yanks it. Therefore, the three solutions I'm considering are the following:

1. I implement the same waiting that occurs in normal mode.

2. Some other shortcut is used, as it is not a feature that does not have a default shortcut in vim.

3. The functionality is left as I had originally intended. Honestly, I don't think there's a problem; after all, if you look at all the sheets applications, they all copy directly to the system clipboard.

I would like to know your opinion on this before proceeding with the implementation.