Require activesupport prior to core_ext/string

When running the test suite with activesupport version 7, an
unitialized constant error for `XmlMini::IsolatedExecutionState`
is raised.

As described here:

- rails/rails#43851
- rails/rails#43852

The resolution for this is to first require `active_support` before
requiring `core_ext/string` so that autoloading happens correctly.

lib/inline_svg.rb

@@ -10,6 +10,7 @@
 require "inline_svg/io_resource"
 
 require "inline_svg/railtie" if defined?(Rails)
+require 'active_support'
 require 'active_support/core_ext/string'
 require 'nokogiri'