Adds snapshot publishing

Changes

Adds a CircleCI config with a deploy-snapshot job that runs for all pushes to main.
Removes the NmcpPlugin, which I had to use in KobanKat. I had created my Maven Central account after a certain date which meant I could only publish via the Central Portal, and com.vanniktech.maven.publish did not have support for that at that time. (See https://github.com/vanniktech/gradle-maven-publish-plugin/issues/722.)
Adjusts the coordinates and POM file to contain RevenueCat info instead of my personal info.
Updates the klib abi dump, because the group changed (to com.revenuecat.purchases).