I eliminated some of the code duplication in the keyup handlers in the browser extension classes by adding the methods `keyCanCreateBlockElement` and `isForbiddenMainContainer` to the editor base to determine if a key code can create a block element and if a tag is a forbidden main container respectively. I think using these methods also makes the keyup handlers a lot more readable.

I also added constants to the core for both of those methods to utilize (`POTENTIAL_BLOCK_ELEMENT_CREATION_KEYS` and `FORBIDDEN_MAIN_CONTAINERS` respectively) to make it easy to modify in future if needed which keys are considered able to create a block element and which tag names are forbidden main containers.