RBS signatures

[drnic]

• Add sig/tiktoken_ruby.rbs signatures
• Introduce steep check
• Run steep check in default rake task
• Run steep check in github ci
• README instructions for libraries using tiktoken_ruby
• Confirm library "tiktoken_ruby" works for client of gem via bundle exec steep check

[drnic]

FWIW this is the first time I've added .rbs sigs into someone else's gem (because I decided to use .rbs in my own WIP gem and figured I'd be helpful :)

[drnic]

NOTE, the BpeFactory signatures weren't required by steep check, but I figured out what I think they should be from the extension.

[IAPark]

Thanks @drnic sorry for never merging this, trying to be more helpful with my own stuff, but still slow. If you'd like to merge this still let me know and I'll reopen this PR