modules: refactor extraFiles

Moved `extraFiles` from `modules/output.nix` into its own file `modules/files.nix`.

Users should now assign text to a `text` attribute, however they could
also assign a file path to a `source` attribute instead.

The old method of directly assigning a string still works, and is
coerced to the new type along with a deprecation warning.

modules/files.nix

@@ -0,0 +1,82 @@
+{
+  lib,
+  helpers,
+  pkgs,
+  ...
+}:
+let
+  fileType = lib.types.submodule (
+    {
+      name,
+      config,
+      options,
+      ...
+    }:
+    {
+      options = {
+        enable = lib.mkOption {
+          type = lib.types.bool;
+          default = true;
+          description = ''
+            Whether this file should be generated.
+            This option allows specific files to be disabled.
+          '';
+        };
+
+        target = lib.mkOption {
+          type = lib.types.str;
+          defaultText = lib.literalMD "the attribute name";
+          description = ''
+            Name of symlink, relative to nvim config.
+          '';
+        };
+
+        text = lib.mkOption {
+          type = with lib.types; nullOr lines;
+          default = null;
+          description = "Text of the file.";
+        };
+
+        source = lib.mkOption {
+          type = lib.types.path;
+          description = "Path of the source file.";
+        };
+      };
+
+      config =
+        let
+          derivationName = "nvim-" + lib.replaceStrings [ "/" ] [ "-" ] name;
+        in
+        {
+          target = lib.mkDefault name;
+          source = lib.mkIf (config.text != null) (
+            # mkDerivedConfig uses the option's priority, and calls our function with the option's value.
+            # This means our `source` definition has the same priority as `text`.
+            lib.mkDerivedConfig options.text (pkgs.writeText derivationName)
+          );
+        };
+    }
+  );
+
+  # TODO: Added 2024-07-07, remove after 24.11
+  # Before we had a fileType, we used types.str.
+  coercedFileType = helpers.transitionType lib.types.str (text: { inherit text; }) fileType;
+in
+{
+  options = {
+    extraFiles = lib.mkOption {
+      type = lib.types.attrsOf coercedFileType;
+      description = "Extra files to add to the runtime path";
+      default = { };
+      example = lib.literalExpression ''
+        {
+          "ftplugin/nix.lua".text = '''
+            vim.opt.tabstop = 2
+            vim.opt.shiftwidth = 2
+            vim.opt.expandtab = true
+          ''';
+        }
+      '';
+    };
+  };
+}