Implemented multi-threaded parsing

[DavidBakerEffendi]

This benefit is not felt on MRI Ruby, as it is locked to a single thread, however JRuby benefits from this immensely.

# Native (MRI) Ruby: Single threaded before PR
bundle exec exe/ruby_ast_gen -i ~/Workspace/test-repos/gitlab  125.88s user 8.24s system 93% cpu 2:23.46 total
# Native (MRI) Ruby: Multi-threaded after PR
bundle exec exe/ruby_ast_gen -i ~/Workspace/test-repos/gitlab  134.01s user 11.44s system 98% cpu 2:28.05 total


# JRuby: This PR (max 10 threads 1 file per thread)
java -jar jruby.jar -S vendor/bundle/jruby/3.1.0/bin/bundle exec  -i   539.63s user 20.85s system 530% cpu 1:45.64 total
# JRuby: Batches of 10 files per thread
java -jar jruby.jar -S vendor/bundle/jruby/3.1.0/bin/bundle exec  -i   544.80s user 21.30s system 495% cpu 1:54.19 total