refactor 'onDispose' of observable disposable to be 'Event'

[legomushroom]

The current implementation would invoke the provided callback immediately if an object is already disposed:

vscode/src/vs/base/common/observableDisposable.ts

Line 32 in 5974364

public onDispose(callback: () => void): IDisposable {

This is different from the default behaviour of the Event object, hence we need to make sure they are consistent.

See #248357 for more details.