This PR makes Prism warn `**` interpreted as argument prefix. This carries a similar meaning to the following Ruby warning: ```console $ ruby -cwe "foo **bar" -e:1: warning: `**' interpreted as argument prefix Syntax OK ``` Previously, it did not issue a warning: ```console $ bundle exec ruby -rprism -ve "p Prism.parse('foo **bar').warnings" ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22] [] ``` From now on, it will issue a warning similar to Ruby's: ```console $ bundle exec ruby -rprism -ve "p Prism.parse('foo **bar').warnings" ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22] [#<Prism::ParseWarning @type=:ambiguous_prefix_star_star @message="ambiguous `**` has been interpreted as an argument prefix" @location=#<Prism::Location @start_offset=4 @length=2 start_line=1> @level=:verbose>] ``` https://github.com/ruby/prism/commit/f6cb5c314c