drop `extensible` property support as never used for now
drop `extensible` property support as never used for now

smelukov/basisjs,istrel/basisjs,istrel/basisjs,fateevv/basisjs,tyanas/basisjs,smelukov/basisjs,basisjs/basisjs,istrel/basisjs,basisjs/basisjs,tyanas/basisjs,fateevv/basisjs,tyanas/basisjs,fateevv/basisjs,smelukov/basisjs,basisjs/basisjs