Fix Racc::ParseError so Pattern Matching is supported

Overview

When using Flog on a project that uses Ruby Pattern Matching, I get the following error:

 bkuhlmann@vilya  rubysmith  alchemists.io  ⑂ release +    flog
ERROR: parsing ruby file ./lib/rubysmith/cli/shell.rb
ERROR! Aborting. You may want to run with --continue.
Traceback (most recent call last):
	23: from /Users/bkuhlmann/.gem/ruby/2.7.2/bin/flog:23:in `<main>'
	22: from /Users/bkuhlmann/.gem/ruby/2.7.2/bin/flog:23:in `load'
	21: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/bin/flog:5:in `<top (required)>'
	20: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog_cli.rb:28:in `run'
	19: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog_cli.rb:162:in `flog'
	18: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog.rb:177:in `flog'
	17: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog.rb:177:in `each'
	16: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog.rb:182:in `block in flog'
	15: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog.rb:194:in `flog_ruby'
	14: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/flog-4.6.4/lib/flog.rb:224:in `flog_ruby!'
	13: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser.rb:33:in `process'
	12: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser.rb:33:in `each'
	11: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser.rb:36:in `block in process'
	10: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser_extras.rb:1317:in `process'
	 9: from /Users/bkuhlmann/.rubies/ruby-2.7.2/lib/ruby/2.7.0/timeout.rb:110:in `timeout'
	 8: from /Users/bkuhlmann/.rubies/ruby-2.7.2/lib/ruby/2.7.0/timeout.rb:33:in `catch'
	 7: from /Users/bkuhlmann/.rubies/ruby-2.7.2/lib/ruby/2.7.0/timeout.rb:33:in `catch'
	 6: from /Users/bkuhlmann/.rubies/ruby-2.7.2/lib/ruby/2.7.0/timeout.rb:33:in `block in catch'
	 5: from /Users/bkuhlmann/.rubies/ruby-2.7.2/lib/ruby/2.7.0/timeout.rb:95:in `block in timeout'
	 4: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser_extras.rb:1329:in `block in process'
	 3: from (eval):3:in `do_parse'
	 2: from (eval):3:in `_racc_do_parse_c'
	 1: from /Users/bkuhlmann/.gem/ruby/2.7.2/gems/ruby_parser-3.15.0/lib/ruby_parser_extras.rb:1304:in `on_error'
/Users/bkuhlmann/.gem/ruby/2.7.2/gems/racc-1.5.1/lib/racc/parser.rb:538:in `on_error': ./lib/rubysmith/cli/shell.rb:27 :: parse error on value ["in", 27] (kIN) (Racc::ParseError)

Steps to Recreate

To recreate, I'm using Flog to illustrate since Flog has Racc as a dependency. Save the following to snippet.rb and run as ruby snippet.rb to see the issue:

# frozen_string_literal: true

require "bundler/inline"

gemfile true do
  source "https://rubygems.org"
  gem "flog", "~> 4.6"
end

collection = {
  a: 1,
  b: 2
}

case collection
  in a: value then puts value
  in b: value then puts value
end

Desired Behavior

Would like to see Racc handle pattern matching without error.

Environment

ruby -v
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]

Start by saving the supplied reproduction as snippet.rb and running it with Ruby 2.7.2 to confirm the Racc::ParseError around the pattern-matching in syntax. Trace how Racc handles this input and add coverage for the example; done means the snippet parses without the reported error.