formula: add .keepme files to prevent cleanup

Allow the use of `.keepme` files inside a keg to prevent the cleanup.

Rather than having a binary state of "never cleanup if `.keepme` is
present" and "can cleanup without", instead a `.keepme` file is
essentially providing reference counting.

It can contain one or more lines which reference files on disk. If the
file exists, the `.keepme` file will prevent cleanup. If/when it does
not: `brew cleanup` will happily cleanup this keg (providing all other
conditions apply).

Library/Homebrew/formula.rb

@@ -3174,6 +3174,7 @@ def system(cmd, *args)
     end
   end
 
+  sig { params(quiet: T::Boolean).returns(T::Array[Keg]) }
   def eligible_kegs_for_cleanup(quiet: false)
     eligible_for_cleanup = []
     if latest_version_installed?
@@ -3201,6 +3202,12 @@ def eligible_kegs_for_cleanup(quiet: false)
             opoo "Skipping (old) #{keg} due to it being linked" unless quiet
           elsif pinned? && keg == Keg.new(@pin.path.resolved_path)
             opoo "Skipping (old) #{keg} due to it being pinned" unless quiet
+          elsif (keepme = keg/".keepme") && keepme.exist? && keepme.readable? &&
+                (keepme_refs = keepme.readlines
+                                     .map(&:strip)
+                                     .filter_map {|line| line if Pathname(line).exist? }
+                                     .presence)
+            opoo "Skipping #{keg} as it needed by #{keepme_refs.join(", ")}" unless quiet
           else
             eligible_for_cleanup << keg
           end