Add smart_quotes and special_strings extensions for Org

Org mode makes a distinction between smart parsing of quotes, and smart
parsing of special strings like `...`. The finer grained control over
these features is necessary to truthfully reproduce Emacs Org mode
behavior. Special strings are enabled by default, while smart quotes are
disabled.

The behavior of `special_string` is brought closer to the reference
implementation in that `\-` is now treated as a soft hyphen.

pandoc-lua-engine/test/lua/module/pandoc-format.lua

@@ -42,6 +42,8 @@ return {
         fancy_lists = false,
         gfm_auto_identifiers = false,
         smart = false,
+        smart_quotes = false,
+        special_strings = true,
         task_lists = true,
       }
       assert.are_same(format.extensions 'org', org_default_exts)