We need a proper lexer to be able to understand YouTube's code, which contains /* inside of strings. For now it's sufficient to just disable comment support altogether. Fixes #4976, fixes #4979, fixes #4980, fixes #4981, fixes #4982. Closes #4977.
In preparation for #4822, extend jsinterp by a lot. (We may even have to/want to write a proper interpreter with actual parsing)