React with pi's digits on pi day

[fwcd]

We already generate 🥧 emoji reactions to "Good Morning" messages on pi day, i.e. March 14, maybe we could extend this mechanism to also react with a new digit for each message sent (per channel). One approach for generating these are streaming Spigot algorithms, which we could even move into a separate Swift package, if we have a nice implementation, potentially under an IteratorProtocol interface. Once we have that, we could even add other irrational/transcendental numbers that are easy to generate.