Hoist constants from Sqids.Builder to Sqids

[robhanlon22]

Since Sqids.Builder doesn't do anything with these constants behavior-wise, it makes sense to me to hoist the constants to Sqids so they're easier to see. Since these constants are now in the top-level Sqids class, I've also made Sqids.Builder package-private since it's not meant to be instantiated directly.

I also added a few things to the .gitignore that showed up in my coding environment.