Pipe operator formatting support

[VTimofeenko]

Description

Nix 2.24 (release announcement post on discourse) added support for experimental pipes (RFC 0148).

I don't believe the exact formatting of code with pipe operator is specified in RFC 0148 or RFC 0166; this issue is about nixfmt not throwing an error when it encounters the pipe operator syntax.

Small example input

1 |> builtins.add 2

Expected output

1 |> builtins.add 2

Actual output

  |
1 | 1 |> builtins.add 2
  |   ^
unexpected '|'
expecting "or", '.', end of input, operator, or term

Steps to reproduce

❯ nix shell "github:NixOS/nixfmt" "github:NixOS/nix?ref=2.24.1"
❯ echo "1 |> builtins.add 2" | nix eval --extra-experimental-features pipe-operators --file -
3
❯ echo "1 |> builtins.add 2" | nixfmt 
<stdin>:1:3:
  |
1 | 1 |> builtins.add 2
  |   ^
unexpected '|'
expecting "or", '.', end of input, operator, or term

[VTimofeenko]

I managed to completely miss #227