fix #3157: register single onDidChangeTextDocument handler and delegate to appropriate mode handler

[jpoon]

What this PR does / why we need it:
This seemed to do the trick to fix #3157. Instead of having an event handler for each modehandler (ie. text editor), declared a single event handler and delegated the event to the right mode handler. This should have the added benefit of improving perf.

To be perfectly honest however, I'm not entirely sure how this fixed the issue as the same callback code to update the position was still being called, albeit unnecessarily for all the other open text editors.

This PR also includes a minor refactor to add a wrapper around registering event handlers to ensure that we are disposing the events properly.

Which issue(s) this PR fixes

#3157

Special notes for your reviewer:

[jpoon]

To be perfectly honest however, I'm not entirely sure how this fixed the issue as the same callback code to update the position was still being called, albeit unnecessarily for all the other open text editors.

Yep, this assessment was correct. This PR doesn't actually fix the issue. Turns out I was using the wrong steps to repro. Once you exit and re-enter insert mode, the issue goes away.

Keeping this PR though as I think it's still valid to just have one event handler instead of one per mode handler.