Add a Stdlib::File::Content type alias

This should describe the valid input for content. It doesn't list Undef
so module authors can enforce content is set. If it's optional,
Optional[Stdlib::File::Content] is easy to use.

spec/type_aliases/file_content_spec.rb

@@ -0,0 +1,29 @@
+# coding: utf-8
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Stdlib::Filemode' do
+  describe 'valid content' do
+    [
+      '',
+      'abc',
+      sensitive('secret'),
+      # TODO: test Deferred?
+    ].each do |value|
+      describe value.inspect do
+        it { is_expected.to allow_value(value) }
+      end
+    end
+
+    context 'with garbage inputs' do
+      [
+        nil,
+        1,
+      ].each do |value|
+        describe value.inspect do
+          it { is_expected.not_to allow_value(value) }
+        end
+    end
+  end
+end

types/file/content.pp

@@ -0,0 +1,2 @@
+# @summary Validate a file content attribute
+type Stdlib::File::Content = Variant[String, Sensitive[String], Deferred[String]]