translation: mark vim.pot as binary

translation: mark vim.pot as binary

`vim.pot` is included in the repository after

```gitcommit
commit 59bd74ed4c9ab366182c93bdc430b186729abbad
Author: Christian Brabandt <cb@256bit.org>
Date:   Sun Jul 13 08:26:57 2025 +0200

    translation: include vim.pot in the repository
```

And it adds quite a lot of noise to the diffs since then.  See the
reasoning in a comment in `.gitattributes`.

I'm not 100% sure that marking it as binary would have no negative side
effects.  But I was not able to find a better option in `git help
attributes`.

Solution suggested in

```gitcommit
commit 5d552d652b0197063565ab937d30f92a9ed28545
Author: Christian Brabandt <cb@256bit.org>
Date:   Tue Jul 15 20:42:48 2025 +0200

    translation: ignore vim.pot creation date, regenerate it, rm allfiles

    Signed-off-by: Christian Brabandt <cb@256bit.org>
```

does not seem to be solving the problem.  It only hides the
`POT-Creation` line from the `vim.pot` diff.  Maybe a more elaborate
filter could be used - one that replaces lines numbers in `vim.pot` with
`xxxx`, thus removing the most annoying and useless part of the diff.

One downside is that it requires everyone to install such a filter
locally - it can not be part of the repo config, as far as I understand.

closes: #17775

Signed-off-by: Illia Bobyr <illia.bobyr@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>