JS codegen: Troubles with strings containing character codes
```yaml
meta:
  id: aaaa
  encoding: utf-8
  endian: le
seq:
  - id: a
    type: str
    size: 4
  - id: b
    type:
      switch-on: a
      cases:
       '"a\0a"': u4
```

causes
```
Parse error: undefined
Call stack: undefined SyntaxError: "0"-prefixed octal literals and octal escape sequences are deprecated; for octal literals use the "0o" prefix instead
```

because ```\000``` is not a valid literal for JS.
