Content of ''\${...} is parsed as Nix

[yorickvP]

rnix-parser seems to be misparsing the ''\${ escape in a multiline string like it's a ${.

Example of formatting inside string
This changes the semantics significantly, with great potential for breakage. It also fails if the contents of the escape happen to not be valid nix.

repl with unformatted and formatted output:

nix-repl> ''''\${[1
          2]}''
"${[1\n2]}"

nix-repl> ''''\${
              [
                1
                2
              ]
            }''
"${\n    [\n      1\n      2\n    ]\n  }"

A related bug is a parse failure on the (valid) ''''\${''