feat(commands): make :x behave more like in vim

[reneleonhardt]

Changes

• Separate :x implementation: Write only if named and modified (but not externally), then quit.
• Does not accept a path anymore (when it was an alias of :write-quit)
• Does not use force (even if given by :x!)
• Had to add Document::externally_overwritten()

Closes #4087

[the-mikedavis]

I merged in #11849 which only checks for modification. I'm not sure checking that the document is named is correct here. Inserting into an unnamed buffer and then trying to exit with :x or :x! in Vim fails.