modules/output: refactor config generation

The motivation for this change was to avoid generating empty
config sections like

    vim.cmd([[

    ]])

To make a config generation cleaner several helper functions introduced:

* `hasContent` have been moved to helpers
* `concatNonEmptyLines` joins strings (which has content) separated with
  newlines
* `wrapVimscriptForLua` wraps a lua string for using in Vimscript, but
  only if the string has content, otherwise empty string is returned
* `wrapLuaForVimscript` wraps Vimscript for using in lua, but only if
  the string has content, otherwise empty string is returned

Added tests:

* testing that all possible config sections are present in the final
  generated config
* testing that the config files generated by empty `files` definitions
  don't have any content in it

modules/top-level/output.nix

@@ -107,16 +107,10 @@ with lib;
         }
       );
 
-      customRC =
-        let
-          hasContent = str: (builtins.match "[[:space:]]*" str) == null;
-        in
-        (optionalString (hasContent neovimConfig.neovimRcContent) ''
-          vim.cmd([[
-            ${neovimConfig.neovimRcContent}
-          ]])
-        '')
-        + config.content;
+      customRC = helpers.concatNonEmptyLines [
+        (helpers.wrapVimscriptForLua neovimConfig.neovimRcContent)
+        config.content
+      ];
 
       init = helpers.writeLua "init.lua" customRC;