CVE-2020-14000: Deserialization of Untrusted Data

MIT Lifelong Kindergarten Scratch scratch-vm loads extension URLs from untrusted project.json files with certain _ characters, resulting in remote code execution because the URL content is treated as a script and is executed as a worker. The responsible code is getExtensionIdForOpcode in serialization/sb3.js. The use of _ is incompatible with a protection mechanism in older versions, in which URLs were split and consequently deserialization attacks were prevented.