[ruby/rdoc] Improve how gemspec's files are defined

(https://github.com/ruby/rdoc/pull/1212)

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
    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
    ```

https://github.com/ruby/rdoc/commit/ac2a151f10

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