feat: An eventHandlerTransformer to control how multiple event handlers emit their values

**Description**

Hello, I ported my blocs to the new `on()` API and things mostly worked :) However I was also affected by the issue raised in #2790 since my blocs expect event handling to be done sequentially. Setting all handlers to sequential() wasn't enough because handling *between multiple event handlers* is always done concurrently (right?)

The suggested solution to use a single `on()` handler with a sequential transformer worked, but it does take away from the elegance of the new API imo

**Proposal**

It would be cool if the event handling order could also be configured. Perhaps an `eventHandlerTransformer` (that gets passed into the constructor?) which lets you configure whether two or more event handlers are run concurrently, sequentially, etc.

So for example, a `Reload` event handler could be droppable, so multiple `Reload` events added in quick succession will only process the first one and drop the succeeding ones. But on a bloc level, inter-event handling could be sequential, so multiple `Reload` events followed by an `Upvote` event in quick succession will result in one reload that finishes (others dropped), followed by the handling of the upvote event after the reload.