Adjust Automatic Date Updates to Run on Save

[farmerau]

Description

Per #221 , on-change events can be triggered twice due to the implementation of the change detection logic.

This is caused by gray-matter appending a newline character when rendering front matter content when the original file doesn't have a newline character, per: https://github.com/jonschlinkert/gray-matter/blob/master/lib/stringify.js#L51 . This is out of user control, which means that in all cases the extension will double-update the last modified property.

The effort here removes the change detection logic altogether, instead using the onWillSaveTextDocument event as a proxy for when we should automatically update the last modified date. This is similar to how a file system might update the last modified property of the file on the file system, which makes sense.

Testing

• I validated that the new logic updates the last modified property as expected on save, when the auto-update setting is enabled.
• I validated that the new logic continues to not update the last modified property as expected when the setting is disabled.
• I validated that other features that update the article continue to function (like editing the title or description in the document)

[estruyf]

Thanks @farmerau this is amazing! Great work! I've also included a check to exclude the new line when it is added by gray matter. This completes the whole functionality.