BUILTIN_ASSET_MIGRATION_SEQUENCE_PREFIX

As ASSET_MIGRATION_SEQUENCE_PREFIX, for asset types **this library ships**.

Same reasoning as BUILTIN_SHAPE_MIGRATION_SEQUENCE_PREFIX: registering com.tldraw.asset.image at version 1 would claim the reference implementation's migration line, and every file it wrote — whose image asset sequence is further along — would fail to load as "data from a newer version". An app's own asset types keep the default prefix, which nothing else claims.