Switch to onWillSaveTextDocument

[SamVerschueren]

The latest insider build has this new event called onWillSaveTextDocument. This allows us to return a TextEdit array which will be applied to the document.

I already made the switch in my vscode-final-newline plugin. Works really great and this also works with the Save All option, which isn't the case with the current implementation.

I will have a look when I find the time.

[jednano]

Sounds excellent!

[SamVerschueren]

The onWillSaveTextDocument has more information regarding the source of the save action (e.g. autosave, manual save, etc.). Maybe we should only apply the transformations when the file is being saved manually? This might fix #58. Although not sure if that's a good strategy because people who use autosave might never press cmd+s themselves.