Formatter cannot format heredoc having interpolation of string literal

[makenowjust]

Extracted from #7099 (comment)

This code causes an error.

<<-FOO
foo#{"foo"}
FOO

$ crystal tool format -< foo.cr
Error: couldn't format STDIN, please report a bug including the contents of it: https://github.com/crystal-lang/crystal/issues

expecting DELIMITER_END, not `INTERPOLATION_START, foo`, at :2:1 (Exception)
  from Crystal::Formatter#check<Symbol>:Nil
  from Crystal::Formatter#visit_string_interpolation<Crystal::StringInterpolation, Crystal::Token, Int32, Int32, Int32, Int32, Bool>:Bool
  from Crystal::Formatter#next_token:Crystal::Token
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::Formatter>:(Bool | Nil)
  from Crystal::format<String>:String
  from Crystal::Command#tool:(Bool | Nil)
  from Crystal::Command#run:(Bool | Crystal::Compiler::Result | Nil)
  from __crystal_main
  from main

[makenowjust]

It is another version, but error message is different.

<<-FOO
#{"foo"}
FOO

Syntax error in :4: Unterminated string literal