formula: add keg_only_reason to formula hash

Library/Homebrew/formula.rb

@@ -1942,6 +1942,7 @@ class Formula
       "version_scheme"           => version_scheme,
       "bottle"                   => {},
       "keg_only"                 => keg_only?,
+      "keg_only_reason"          => keg_only_reason&.to_hash,
       "bottle_disabled"          => bottle_disabled?,
       "options"                  => [],
       "build_dependencies"       => dependencies.select(&:build?)

Library/Homebrew/formula_support.rb

@@ -60,6 +60,19 @@ class KegOnlyReason
       @reason
     end.strip
   end
+
+  def to_hash
+    reason_string = if @reason.is_a?(Symbol)
+      @reason.inspect
+    else
+      @reason.to_s
+    end
+
+    {
+      "reason"      => reason_string,
+      "explanation" => @explanation,
+    }
+  end
 end
 
 # Used to annotate formulae that don't require compiling or cannot build a bottle.