Editor: Move core blocks registration to init hook

After working on support for register_block_type_args filter in #49615, it became clear that we need to use init action for core blocks to make it possible to use this filter.

Fixes #50263.


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git-svn-id: https://core.svn.wordpress.org/trunk@48048 1a063a9b-81f0-0310-95a4-ce76da25c4cd