Hoist constants from Sqids.Builder to Sqids

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.

.gitignore has also been updated to ignore the .project file that was
generated by my dev environment.

.gitignore

@@ -1,4 +1,5 @@
 .gradle/
+.project
+.vscode/
 bin/
 build/
-.vscode/