Fix invalid bytes sequence in UTF-8 error

[mallowlabs]

I found invalid byte sequence in UTF-8 error error is raised with my source code.
If source contains byte sequence string literal like "\xff" and rule uses regexp like /foo/, the error raises.
This PR will fix it.

querly.yml

rules:
  - id: do_not_use_foo
    pattern:
      subject: ":string: as 'str"
      where:
        str: /foo/
    message: Do not use foo.

target.rb

"\xff"

[soutaro]

Thanks @mallowlabs! 🎉