Rake entrypoint registers Bootsnap's cache_dir for cleaning

The rake entrypoint also allows future rake tasks to be exposed, should
that be necessary. Expected use is via: `require "bootsnap/rake"` from
users' Rakefile.

Additionally, should future rake tasks be provided by bootsnap, the
expectation is that they would live under `lib/bootsnap/tasks/*.rake`
such that user's can add Bootsnap's tasks via: `Rake.add_tasklib`.

The expectation is that rake.rb would contain the task _classes_, and
any tasks/*.rake would define them as tasks. Then users could require
bootsnap/rake to construct their own tasks using the classes, or
add_tasklib to use the provided tasks.

Author: jasonkarns

lib/bootsnap/rake.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require "bootsnap"
+require "rake/clean"
+
+# Typically, should have been set up prior to requiring rake integration.
+# But allow for a streamlined ::default_setup
+require "bootsnap/setup" if Bootsnap.cache_dir.nil?
+
+if Bootsnap.cache_dir
+  CLEAN.include Bootsnap.cache_dir
+else
+  abort "Bootsnap must be set-up prior to rake integration"
+end