chore: remove ruby-next-core dependency

Modify LOAD_PATH manually and use a single set of transpiled files for all EOL Rubies

.rbnextrc

@@ -1,3 +1,4 @@
 nextify: |
   ./lib
   --min-version=2.2
+  --single-version

lib/paco.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
-require "ruby-next/language/setup"
-RubyNext::Language.setup_gem_load_path(transpile: true)
+if RUBY_VERSION < "2.6.0"
+  $LOAD_PATH.unshift File.expand_path("./.rbnext", __dir__)
+end
 
 require "paco/version"
 require "paco/parse_error"

paco.gemspec

@@ -26,11 +26,5 @@ Gem::Specification.new do |spec|
 
   spec.require_paths = ["lib"]
 
-  # When gem is installed from source, we add `ruby-next` as a dependency
-  # to auto-transpile source files during the first load
-  if ENV["RELEASING_PACO"].nil? && File.directory?(File.join(__dir__, ".git"))
-    spec.add_runtime_dependency "ruby-next", ">= 0.15.0"
-  else
-    spec.add_runtime_dependency "ruby-next-core", ">= 0.15.0"
-  end
+  spec.add_development_dependency "ruby-next", ">= 0.15.0"
 end