Change "Delete active text file"

I think we can agree the terminology used in the popup of the text editor's × button should be accurate and reflect the underlying behavior to avoid confusion. And technically, both Unlink and Delete are accurate, while I'd argue that Close is quite further away and dangerously misleading.

To remind the context in Blender:

1. Any data that is unused is automatically deleted when you close a file, reload it ("revert"), or run a data purge in the outliner. We can also mark any data with a "fake user", so that no matter what happens to that data, it won't get automatically deleted.
2. Most data types are often used by at least one user (materials, textures, ...). But some data can easily be left with no users, such as brushes, and text data.
3. Text data have the particularity of being used basically nowhere outside the text editor, hence by default they are all marked with a fake user so to not get lost automatically.
4. Blender can contain data that doesn't exist outside of Blender. Any deletion of such data is permanent with no way of recovery, so you can argue it is a good thing for the UX to be overly cautious when dealing with data deletion.

In that context:

1. "Unlink" is technically correct : Clicking × unlinks the text data from the editor, meaning it is no longer associated with the editor but remains in memory. If you save or close Blender without marking the data with a fake user, it will be lost. This is consistent with Blender's general data-block system.

2. "Delete" is also accurate, but with a different nuance : If not marked with a fake user, that data will eventually be gone forever. However, "delete" could confuse users who assume the data will immediately and permanently disappear, which is incorrect: text data is most often marked with a fake user, and even if the user unmarks them, it still won't be deleted immediately.

3. "Close" is misleading : "Close" suggests that the data remains safely stored, like closing a document in other software. This term ignores the potential for data loss and fails to communicate the need for user caution to preserve the text data.