fix: enable-url-completion input treating the string "false" as true (#307)

* NO-ISSUE Add test

* NO-ISSUE Use core.getBooleanInput() to retrieve boolean input values

* NO-ISSUE npm run build

* NO-ISSUE Delete unused import

action.yml

@@ -22,6 +22,7 @@ inputs:
     description: >
       Enables completion of relative URLs to absolute ones
       Default: `false`
+    default: "false"
   image-extensions:
     description: >
       File extensions that will be treated as images