Multiline R strings are not correctly highlighted

**Information** - Language: R - Plugins: autoloader **Description** Multiline R strings are not correctly highlighted. Similar to #457 for CSS/JS or #1233 for PHP. **Code snippet** [Test page](https://prismjs.com/test#language=r&text=x%20%3C-%20%22multi%0Aline%0Astring%22%0A%0Ay%20%3C-%20%22%0Aanother%20one%0A%22) <details> <summary>The code being highlighted incorrectly.</summary> ``` x <- "multi line string" y <- " another one " ``` </details>