Escape ${ in strings when printing Nix expressions

[soulomoon]

Fix for #1000
Following the fix in NixOS/nix 4012

• What is done:

  • stop escaping a single $ sign in double quoted string if not following by { when printing.
  • escape ${ to \${ in double quoted string when printing.
  • update the tests for this behavior.
  • refactor to drop the usage of show in printNix, and unify some double string escape behavior for prettyString and printNix

• What is not working:

  • test eval-okay-ind-string is malformed since the nix submodule is outdated(the test violate NixOS/nix 4012), however the test does work for Nix 2.4 version of eval-okay-ind-string.exp, this test should be banned until modern Nix submodule is pulled Please, use modern Nix submodule #610

[soulomoon]

Then I have done a refactoring of escape :: Char -> [Char] because I saw the comment.
Since we need to identify two Char pattern, the work around making the code ugly.

There is a way to refactor the code into nicer version by doing explicit recursion using bidirectional pattern synonym.

The idea is coming from here

[Anton-Latukha]

Checking the code locally for now (upstream shenanigans are currently not allowed to make GHC 9.0 CI usable).

[Anton-Latukha]

Reviewed the code.

Works locally.

[Anton-Latukha]

Thank you for this