Rename sub directory for installation stores

[seratch]

Summary

Now that we have state-stores directory for StateStore, renaming stores to installation-stores would be better in terms of consistency.

As long as users import the interface / classes from @slack/oauth, this change does not affect any existing apps. Thus, I don't think that we can this a breaking change. What do you think?

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[filmaj]

Agreed, this is fine. Breaking change would only apply to public APIs; importing specific package source files from sub directories is never recommended for exactly this reason.

[seratch]

@filmaj Thanks for your quick response!