Add Gem RBI again

[connorshea]

You'll note that the branch is the same as #92 (but with one extra commit to remove some untyped sigs I missed last time).

The reason we reverted #92 was because of the issue reported in #105. However, I'm able to reproduce that problem without any gem.rbi. The problem happened to be noticed at the same time as gem.rbi was merged in, but it's entirely unrelated.

The problem is caused by bundler.rbi being changed from true to strong, presumably because srb rbi sorbet-typed sets all the sorbet-typed RBIs to strong and doesn't run suggest-typed.

You can reproduce this in any project by changing the sigil in bundler.rbi to anything above true. The issue is fixed by running suggest-typed to update the type sigils in all the files in a project.

You can see this in my project, where I made a commit that changed bundler.rbi to typed: strong. It was passing before, and failed with the issue described in #105 afterward. gem.rbi does not exist in either commit.

srb tc job log, no gem.rbi, bundler.rbi set to typed: true: https://gitlab.com/connorshea/VideoGameList/-/jobs/269507607

srb tc job log, no gem.rbi, bundler.rbi set to typed: strong: https://gitlab.com/connorshea/VideoGameList/-/jobs/269956305

[elliottt]

Thanks for sorting out what happened in #105!