Add config option to use project root as shellcheck working dir

`false` is default, which maintains the plugin's existing behavior

Author: Ted Behling

package.json

@@ -31,6 +31,11 @@
       "type": "boolean",
       "title": "Enable Notice Messages",
       "default": false
+    },
+    "useProjectCwd": {
+      "type": "boolean",
+      "title": "Run Shellcheck using project root as the working directory (false for file-relative)",
+      "default": false
     }
   },
   "dependencies": {