Raw string literal causes parsing error

[Steven]

This causes a parsing error in Gluon: r"asdf":

error: unterminated string literal
  ┌─ test:1:1
  │
1 │ ╭ r"asdf"
2 │ │
  │ ╰^

In comparison, this parses:

r"asdf""

The result of which is "asdf\"".

This is tested on v0.17.1, on commit e150b9f8, so in terms of parsing is master.