Add squish filter

lib/liquid/standardfilters.rb

@@ -293,6 +293,19 @@ module Liquid
       input.split(pattern)
     end
 
+    # @liquid_public_docs
+    # @liquid_type filter
+    # @liquid_category string
+    # @liquid_summary
+    #   Removes leading and trailing whitespace and collapses consecutive whitespace to a single space.
+    # @liquid_syntax string | squish
+    # @liquid_return [string]
+    def squish(input)
+      return if input.nil?
+
+      Utils.to_s(input).strip.gsub(/\s+/, ' ')
+    end
+
     # @liquid_public_docs
     # @liquid_type filter
     # @liquid_category string