Add a nix flake for building with nix (#185)

Also based on #165

---------

Co-authored-by: o1lo01ol1o <tim.pierson@gmail.com>
Co-authored-by: Tim Pierson <o1lo01ol1o@users.noreply.github.com>

nix/expose-all.patch

@@ -0,0 +1,12 @@
+diff --git a/gf.cabal b/gf.cabal
+index 0076e7638..8d3fe4b49 100644
+--- a/gf.cabal
++++ b/gf.cabal
+@@ -168,7 +168,6 @@ Library
+     GF.Text.Lexing
+     GF.Grammar.Canonical
+
+-  other-modules:
+     GF.Main
+     GF.Compiler
+     GF.Interactive