Create nix-ts-mode sub-directory; Add additional snippets to nix-mode (#515)

* nix-ts-mode: New snippet directory.

* nix-mode: pkg_mkshell: New snippet

This snippet is designed to quickly write out a `shell.nix` for Nix.

* nix-mode: doCheck: New snippet.

Adds a snippet to add the doCheck attribute to a package derivation
that uses the current standard: a condition for cross-compilation or
false.

* nix-mode: passthru_update: New snippet.

This adds a snippet for adding the `passthru.updateScript` attribute
used in many nixpkgs derivations.

Author: normalcea

snippets/nix-mode/doCheck

@@ -0,0 +1,8 @@
+# -*- mode: snippet -*-
+# name: doCheck
+# key: dc
+# --
+doCheck = ${1:$$(yas-auto-next
+                  (yas-choose-value
+                  '("stdenv.buildPlatform.canExecute stdenv.hostPlatform;"
+                    "false;")))}

snippets/nix-mode/passthru_update

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: passthru_update
+# key: pu
+# --
+passthru.updateScript = nix-update-script { };

snippets/nix-mode/pkgs_mkshell

@@ -0,0 +1,13 @@
+# -*- mode: snippet -*-
+# name: pkgs_mkshell
+# key: pms
+# --
+pkgs.${1:$$(yas-auto-next (yas-choose-value '("mkShell" "mkShellNoCC")))} {
+  inputsFrom = [ $3 ];
+
+  packages = [ $2 ];
+
+  shellHook = ''
+  $4
+  '';
+}$0

snippets/nix-ts-mode/.yas-parents

@@ -0,0 +1 @@
+nix-mode