Add simple RBS signatures for generated files

[joker1007]

This pull request add rbs signatures for generated files.

Union type and type alias are not supported currently. I handle them as untyped now.

nevertheless, These signatures are useful for type checking and code completion.

I also write Steepfile to enable type checking.
But, I met a mysterious DuplicatedDeclarationError.
Because I couldn't resolve this, some type-checking errors are left.

[mtsmfm]

Thanks.
Can you update gemspec to include the sig dir?
https://github.com/ruby/rbs/blob/3e76a4d743b31542d417be563a25d7c54c0e2a76/docs/gem.md#sig-directory

[joker1007]

@mtsmfm
I added sig directory to spec.files.

[mtsmfm]

@joker1007 Can you rebase this branch onto the latest main? I'd like to see the result for generator workflow and CI failure should be fixed.

[joker1007]

@mtsmfm
OK. I rebased this.

[mtsmfm]

Suggested change

	gem "steep"
	gem "steep", require: false

To fix this failure

https://github.com/mtsmfm/language_server-protocol-ruby/actions/runs/9206138738/job/25323641047

I know it's already retired though I'd like to not drop old rubies until we face huge difficulties just for my curiosity

[mtsmfm]

@joker1007 Can you apply this change?

[mtsmfm]

@joker1007 Poke

[joker1007]

I'm sorry. I forgot to apply the change.
I fixed it.

[mtsmfm]

Sorry for bothering you, could you rebase this branch again?

Hopefully this change fixes the generator CI for forked repos

#106

[joker1007]

No problem!
I rebased this.

[mtsmfm]

Thanks!