Introduce NotebookDocumentChangeEvent#reason

[rebornix]

Had a discussion with @dbaeumer and learnt that adding reason was perhaps not sufficient or not what was required.
Salient points of the meeting:

• Existing event is unfortunately a little complex and difficult to use/understand, as this event can be triggered for a number of reasons related to notebook content and notebook execution state changes.
  • Hence consumers will likely make assumptions when consuming this event.
• When consuming this event (at least in the case of LSP)
  • Difficult to understand the current execution state (starting, executing, stopped, etc)
  • Are changes a result of execution or other

A follow up discussion will be required with @dbaeumer and a few others to some of the above issues (& possibly others, if any) are addressed

For #211142