Add a lexer, tests and examples for COBOL

[bartbroere]

In this PR I add a lexer for COBOL, some tests and an example sourced from a Creative Commons repository.

Hopefully this PR is complete enough. Let me know if I should add or fix some things.

Resolves #1569

[bartbroere]

The CI fails for 2.7 because of this:

/home/runner/work/rouge/rouge/lib/rouge/lexers/mojo.rb:6:in `<module:Lexers>': uninitialized constant Rouge::Lexers::Python (NameError)
	from /home/runner/work/rouge/rouge/lib/rouge/lexers/mojo.rb:[5](https://github.com/rouge-ruby/rouge/actions/runs/10759257587/job/29835709356?pr=2067#step:4:6):in `<module:Rouge>'
	from /home/runner/work/rouge/rouge/lib/rouge/lexers/mojo.rb:4:in `<top (required)>'
	from /home/runner/work/rouge/rouge/lib/rouge/lexer.rb:532:in `load'
	from /home/runner/work/rouge/rouge/lib/rouge/lexer.rb:532:in `load_lexer'
	from /home/runner/work/rouge/rouge/lib/rouge.rb:52:in `block in load_lexers'
	from /home/runner/work/rouge/rouge/lib/rouge.rb:51:in `each'
	from /home/runner/work/rouge/rouge/lib/rouge.rb:51:in `load_lexers'
	from /home/runner/work/rouge/rouge/lib/rouge.rb:[6](https://github.com/rouge-ruby/rouge/actions/runs/10759257587/job/29835709356?pr=2067#step:4:7)7:in `<top (required)>'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:60:in `require'
	from /opt/hostedtoolcache/Ruby/2.[7](https://github.com/rouge-ruby/rouge/actions/runs/10759257587/job/29835709356?pr=2067#step:4:8).8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:60:in `block (2 levels) in require'
	from /opt/hostedtoolcache/Ruby/2.7.[8](https://github.com/rouge-ruby/rouge/actions/runs/10759257587/job/29835709356?pr=2067#step:4:9)/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:55:in `each'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:55:in `block in require'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:44:in `each'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler/runtime.rb:44:in `require'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/bundler-2.4.22/lib/bundler.rb:187:in `require'
	from /home/runner/work/rouge/rouge/spec/spec_helper.rb:6:in `<top (required)>'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:83:in `require'
	from /opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:83:in `require'
rake aborted!
Command failed with status (1)
/home/runner/work/rouge/rouge/vendor/bundle/ruby/2.7.0/gems/rake-[13](https://github.com/rouge-ruby/rouge/actions/runs/10759257587/job/29835709356?pr=2067#step:4:14).2.1/exe/rake:27:in `<top (required)>'
/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/bundle:23:in `load'
/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/bundle:23:in `<main>'
Tasks: TOP => check:specs
(See full trace by running task with --trace)

Since it doesn't seem related to my changes, I opened the PR #2068 to fix this.

[tancnle]

suggestion: Could we convert this into a class variable to take advantage of lazy loading? Constants are eagerly loaded when the class is defined, which can slow down startup time. Also, since the collection is rather big, Sets can be more memory-efficient due to their hash-based implementation.

def keywords
  @keywords ||= Set.new(%w(ACCEPT ACCESS ...))
end

[bartbroere]

I applied this, and two of your other suggestions. Hope it is what you meant, Ruby is a relatively new language to me so I had to do some learning about class variables and lazy loading.

[tancnle]

Thank you for your work on this PR, @bartbroere 🙇🏼‍♂️. The changes look good overall. I've left some suggestions. Please let me know what you think 🙏🏼