Fix #1453: App crashes when unsaved file alert presented

[avinizhanov]

Description

I think it crashed because file.canClose had withDelegate: self instead of WorkspaceDocument.
I didn't find an easy way to pass WorkspaceDocument to Editor, so I copied document(_:shouldClose:contextInfo:) to Editor and it helped.

I moved it to separate method canCloseTab, if user in alert select Cancel it will not close tab.

Also when tab close added:
item.fileDocument = nil

Previously it kept file in memory and didn't reload content when you reopened file.

Related Issues

• 🐞 App crashes when unsaved file alert presented #1453

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

Screen.Recording.2023-10-16.at.5.24.22.PM.mov

[austincondiff]

Looks good!