Support code block creation with ```lang + Enter

[ysds]

Description

In most Markdown editors, typing ```lang and pressing Enter creates a code block. Currently, BlockNote only supports ```lang + Space to trigger code block creation, because ProseMirror input rules only fire on text input (handleTextInput), not on Enter key (handleKeyDown).

Expected behavior

Typing ```lang followed by Enter should create a code block with language, just like ```lang + Space does today.

While users could work around this by adding a custom extension with an Enter keyboard shortcut, creating code blocks with ```lang + Enter is a widely expected Markdown editing convention. Providing this as a built-in behavior improves the out-of-the-box experience and avoids requiring every user to implement the same workaround independently.

Additional context

I can submit a PR for this, but it may be related to #2389 and #2345, so holding off for now.