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

tests/test-sources/modules/output.nix

@@ -4,4 +4,90 @@
     # Make sure jsregexp is in LUA_PATH
     extraConfigLua = ''require("jsregexp")'';
   };
+
+  # Test that all extraConfigs are present in output
+  all-configs.module =
+    {
+      config,
+      pkgs,
+      lib,
+      ...
+    }:
+    let
+      configs = {
+        extraConfigLuaPre = "string.format('extraConfigLuaPre1')";
+        extraConfigLua = "string.format('extraConfigLua2')";
+        extraConfigLuaPost = "string.format('extraConfigLuaPost3')";
+        extraConfigVim = "let g:var = 'extraConfigVim4'";
+      };
+      mkConfigAssertions = name: value: [
+        {
+          assertion = lib.hasInfix "extraConfigLuaPre1" value;
+          message = "Configuration file ${name} does not contain extraConfigLuaPre.";
+        }
+        {
+          assertion = lib.hasInfix "extraConfigLua2" value;
+          message = "Configuration file ${name} does not contain extraConfigLua.";
+        }
+        {
+          assertion = lib.hasInfix "extraConfigLuaPost3" value;
+          message = "Configuration file ${name} does not contain extraConfigLuaPost.";
+        }
+        {
+          assertion = lib.hasInfix "extraConfigVim4" value;
+          message = "Configuration file ${name} does not contain extraConfigVim.";
+        }
+      ];
+    in
+    configs
+    // {
+      files = {
+        "test.lua" = configs;
+        "test.vim" = configs;
+      };
+
+      # Plugin configs
+      extraPlugins = [
+        {
+          plugin = pkgs.emptyDirectory;
+          config = "let g:var = 'neovimRcContent5'";
+        }
+      ];
+
+      assertions =
+        mkConfigAssertions "init.lua" config.content
+        ++ mkConfigAssertions "test.lua" config.files."test.lua".content
+        ++ mkConfigAssertions "test.vim" config.files."test.vim".content
+        # Check the final generated init.lua too
+        ++ mkConfigAssertions "initPath" (builtins.readFile config.initPath)
+        ++ [
+          # Only init.lua contains configuration from plugin definitions
+          {
+            assertion = lib.hasInfix "neovimRcContent5" (builtins.readFile config.initPath);
+            message = "Configuration file init.lua does not contain plugin configs";
+          }
+        ];
+    };
+
+  files-default-empty.module =
+    { config, helpers, ... }:
+    {
+      files = {
+        # lua type
+        "test.lua" = { };
+        # vim type
+        "test.vim" = { };
+      };
+
+      assertions = [
+        {
+          assertion = !helpers.hasContent config.files."test.lua".content;
+          message = "Default content of test.lua file is expected to be empty.";
+        }
+        {
+          assertion = !helpers.hasContent config.files."test.vim".content;
+          message = "Default content of test.vim file is expected to be empty.";
+        }
+      ];
+    };
 }