Improve how gemspec's files are defined

Currently, the gemspec's files are defined by hand, which is error-prone.

For example: #1211

This commit uses `Dir.glob` where possible to reduce the risk of that
happening again.

- Additional files added with this approach:

    ```
    # This should have been added by only captured by this commit
    lib/rdoc/parser/prism_ruby.rb

    # These are folders and can be included/ignored either way
    lib/rdoc/generator/template/darkfish
    lib/rdoc/generator/template/darkfish/css
    lib/rdoc/generator/template/darkfish/fonts
    lib/rdoc/generator/template/darkfish/images
    lib/rdoc/generator/template/darkfish/js
    lib/rdoc/generator/template/json_index
    lib/rdoc/generator/template/json_index/js
    ```

- Files that are ignored after this change:

    ```
    # They make no difference on documentation generation
    # Probably can be removed
    lib/rdoc/generator/template/darkfish/.document
    lib/rdoc/generator/template/json_index/.document
    ```

Author: st0012

rdoc.gemspec

@@ -37,7 +37,26 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   s.require_paths = ["lib"]
   # for ruby core repository. It was generated by
   # `git ls-files -z`.split("\x0").each {|f| puts "    #{f.dump}," unless f.start_with?(*%W[test/ spec/ features/ .]) }
-  s.files = [
+  NONE_LIB_FILES = [
+    "CONTRIBUTING.rdoc",
+    "CVE-2013-0256.rdoc",
+    "ExampleMarkdown.md",
+    "ExampleRDoc.rdoc",
+    "History.rdoc",
+    "LEGAL.rdoc",
+    "LICENSE.rdoc",
+    "README.rdoc",
+    "RI.md",
+    "TODO.rdoc",
+    "exe/rdoc",
+    "exe/ri",
+    "man/ri.1",
+  ]
+  TEMPLATE_FILES = Dir.glob("lib/rdoc/generator/template/**/*")
+  LIB_FILES = Dir.glob("lib/**/*.{rb,kpeg,ry}")
+
+  s.files = (NONE_LIB_FILES + TEMPLATE_FILES + LIB_FILES).uniq
+  old_files = [
     "CONTRIBUTING.rdoc",
     "CVE-2013-0256.rdoc",
     "ExampleMarkdown.md",
@@ -225,8 +244,11 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
     "lib/rdoc/version.rb",
     "man/ri.1",
   ]
-  # files from .gitignore
-  s.files << "lib/rdoc/rd/block_parser.rb" << "lib/rdoc/rd/inline_parser.rb" << "lib/rdoc/markdown.rb" << "lib/rdoc/markdown/literals.rb"
+  puts s.files.count
+  puts "-------------------"
+  puts s.files - old_files
+  puts "-------------------"
+  puts old_files - s.files
 
   s.rdoc_options = ["--main", "README.rdoc"]
   s.extra_rdoc_files += s.files.grep(%r[\A[^\/]+\.(?:rdoc|md)\z])