Perform static type checking

[Yang-33]

In v2, we have set up the process to automatically generate RBS files along with the code. For static type checking, there are tools like steep and sorbet. While Sorbet might seem popular(?), speed is not that crucial for us, and it seems there are no special RBS files needed, as the SDK code is not very complex. Therefore, I thought it would be good to start with steep, which is easy to begin with. Switching to aorbet later doesn't seem too difficult.

There are three reasons to do this:

1. To detect any strange code that might be generated and doesn't work at the type level.
2. To automatically verify the consistency between types and implementation. If not verified, it can lead to user confusion.
3. To ensure that examples and tests work correctly at the type level. Examples in dynamically typed languages tend to break easily, which can discourage SDK beginners. This should not be the case.

[Yang-33]

WIP, but almost errors are resolved (or ignored..)