Fixes a few possible memory leaks with decoration registration by registering the returned disposable.

Details

Currently decorations are registered like this:

this.codeEditorService.registerDecorationType(decorationDescription, placeholderDecorationType, {});

But it can be easy to forget that most likely the decoration also needs to be removed on dispose:

this.codeEditorService.removeDecorationType(placeholderDecorationType);

Change

The change is to switch to using this._register, which seems to make it easier to register the decoration as a disposable. Now for example in commentsController or breakPointWidget the decoration registration is registered as a disposable.