Whether a block type is already registered — lets a plugin author check before registering
instead of catching the duplicate-name throw, and lets registerBuiltinBlocks stay idempotent
across the duplicated module copies described above (a module-scoped "done" flag can't).
Whether a block type is already registered — lets a plugin author check before registering instead of catching the duplicate-name throw, and lets registerBuiltinBlocks stay idempotent across the duplicated module copies described above (a module-scoped "done" flag can't).