Add RBS signature and testing

[ksss]

I propose managing the signatures in a ruby/bigdecimal repository instead of ruby/gem_rbs_collection.

Background

bigdecimal is a bundled gem.
Until now, its signatures were managed in the RBS repository, but bundled gems are now expected to be maintained either in gem_rbs_collection or in the gem itself.

Supported signatures

• bigdecimal.rb
  • Support all methods without internal.
• bigdecimal/math
  • Support all methods
• bigdecimal/util
  • Support all methods

Testing of signatures

No modifications have been made to existing Ruby tests.
I have rewritten a new test for signatures.
It is sufficient to test signatures only with the latest version of Ruby.

Comment of signatures

Ruby comments and rbs comments are synchronized using the rake rbs:annotate command.
After operating with ruby/base64, I received feedback that it's not necessary to check comment updates with CI, and that updating them at arbitrary timings seems sufficient.

Benefit

We can provide users with signatures that follow the Ruby code perfectly.