Remove Gemfile, add script for Rubocop

[herwinw]

This addresses the issues from #1180.
I've opted for using bundler/inline, so we can still force a specific version. The script itself is fully compatible with Rubocop, so we can things like bin/rubocop --autocorrect work too.
I've also updated the CI job to Ruby 3.3, it was still using 3.0. It should probably work with every supported Ruby version.

[eregon]

I think you should be able to use require Gem.bin_path('rubocop', 'rubocop') instead to avoid having to copy the logic from exe/rubocop.

[herwinw]

This did result in a LoadError, even though the file does exist (probably because it expects a .rb extension). Using Kernel.exec works.

[eregon]

Right, require is picky about the extension.
load would likely work then.

I think exec might lose the state from bundler above and potentially run a different version of RuboCop, so could you try load?

Alternatively we could exec but pass the version too like:
exec(Gem.bin_path('rubocop', 'rubocop'), '_1.66.1_', *ARGV)
but that probably won't work if bundle path config is set (which I have).

[herwinw]

The bin_path includes the full version in the path (so in my case it's something like ~/.rbenv/versions/3.3.5/lib/ruby/gems/3.3.0/gems/rubocop-1.66.1/exe/rubocop). I've tried it with switching to a different version and running bin/rubocop -v, and this shows the correct version.

[eregon]

You're right it seems to be correct:

$ cat /home/eregon/.rubies/ruby-3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-1.56.3/exe/rubocop 
#!/usr/bin/env ruby
# frozen_string_literal: true

$LOAD_PATH.unshift("#{__dir__}/../lib")
...

because of that $LOAD_PATH modification.
Gems are not supposed to do that but whatever, it helps here.

The ultra correct way would be to replicate what bin/rubocop does:

cat ~/.rubies/ruby-3.2.2/bin/rubocop 
#!/home/eregon/.rubies/ruby-3.2.2/bin/ruby
#
# This file was generated by RubyGems.
#
# The application 'rubocop' is installed as part of a gem, and
# this file is here to facilitate running it.
#

require 'rubygems'

Gem.use_gemdeps

version = ">= 0.a"

str = ARGV.first
if str
  str = str.b[/\A_(.*)_\z/, 1]
  if str and Gem::Version.correct?(str)
    version = str
    ARGV.shift
  end
end

if Gem.respond_to?(:activate_bin_path)
load Gem.activate_bin_path('rubocop', 'rubocop', version)
else
gem "rubocop", version
load Gem.bin_path("rubocop", "rubocop", version)
end

Which explicitly activates the gem with the optionally-given version (and otherwise picks latest).

Anyway, the exec seems fine for now, we can always improve this later if needed.