Refactor issue processor (#45)

* Refctor into an issue processor, add debug mode
* Move processor to use its own types so testing is easier
* Add more tests

action.yml

@@ -33,6 +33,9 @@ inputs:
   operations-per-run:
     description: 'The maximum number of operations per run, used to control rate limiting.'
     default: 30
+  debug-only:
+    description: 'Run the processor in debug mode without actually performing any operations on live issues.'
+    default: false
 runs:
   using: 'node12'
   main: 'dist/index.js'