Add code-block to the list of blocks where hitting return will split-block
Add code-block to the list of blocks where hitting return will split-block

Hitting return in a code-block will now split the block into two code blocks. (Holding shift will still insert a soft newline into the current block).

We still need to make it a bit more obvious that consecutive code-blocks
are not contiguous - https://github.com/vector-im/riot-web/issues/4535

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