Improve how gemspec's files are defined

[st0012]

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

For example: https://github.com/ruby/rdoc/pull/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
  # gems using Dir.glob in gemspec usually have them included
  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

[st0012]

@olleolleolle Thanks, updated 👍