Add support for MultilineStrings extension

[yaitskov]

GHC 9.12 introduced multiline string literals feature.
Triple double quote denotes a String.
After such token indentation breaks a bit.

[webdevred]

Hi,

Thank you very much for reporting the issue with multiline string literals in haskell-mode. I will start working on adding support for the new triple double-quote string syntax introduced in GHC 9.12.

Thanks again for your help!

Best regards,
webdevred