Add "webrick" to Gemfile for Ruby 3 support

[steffen]

Running script/server in a default GitHub codespace yields this error:

/usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve/servlet.rb:3:in `require': cannot load such file -- webrick (LoadError)
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve/servlet.rb:3:in `<top (required)>'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:184:in `require_relative'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:184:in `setup'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:102:in `process'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:93:in `block in start'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:93:in `each'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:93:in `start'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/lib/jekyll/commands/serve.rb:75:in `block (2 levels) in init_with_program'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
        from /usr/local/rvm/gems/ruby-3.1.3/gems/jekyll-3.9.2/exe/jekyll:15:in `<top (required)>'
        from /usr/local/rvm/gems/ruby-3.1.3/bin/jekyll:25:in `load'
        from /usr/local/rvm/gems/ruby-3.1.3/bin/jekyll:25:in `<main>'
        from /usr/local/rvm/gems/ruby-3.1.3/bin/ruby_executable_hooks:22:in `eval'
        from /usr/local/rvm/gems/ruby-3.1.3/bin/ruby_executable_hooks:22:in `<main>'

The reason is that a default GitHub codespace uses Ruby 3 (my codespace used ruby 3.1.3p185 (2022-11-24 revision 1a6b16756e) [x86_64-linux] to be exact, via ruby --version).

The webrick gem is no longer a bundled gem in Ruby 3.0, see jekyll/jekyll#8523.
Therefore I propose to add it to the Gemfile.

I added the entry by running bundle add webrick and confirmed that script/server works afterwards.

[chrisd8088]

Thanks for the patch! This does make things work better in a codespace now, so I'll merge it. Much appreciated!