feat: permissions (#168)

- Load `app-permissions` from schema exported by `@octokit/openapi`
- Update documentation in README.md
- Implement the `permissions_*` inputs in the action code

---------

Co-authored-by: Parker Brown <17183625+parkerbxyz@users.noreply.github.com>

lib/get-permissions-from-inputs.js

@@ -0,0 +1,23 @@
+/**
+ * Finds all permissions passed via `permision-*` inputs and turns them into an object.
+ *
+ * @see https://docs.github.com/en/actions/sharing-automations/creating-actions/metadata-syntax-for-github-actions#inputs
+ * @param {NodeJS.ProcessEnv} env
+ * @returns {undefined | Record<string, string>}
+ */
+export function getPermissionsFromInputs(env) {
+  return Object.entries(env).reduce((permissions, [key, value]) => {
+    if (!key.startsWith("INPUT_PERMISSION_")) return permissions;
+
+    const permission = key.slice("INPUT_PERMISSION_".length).toLowerCase();
+    if (permissions === undefined) {
+      return { [permission]: value };
+    }
+
+    return {
+      // @ts-expect-error - needs to be typed correctly
+      ...permissions,
+      [permission]: value,
+    };
+  }, undefined);
+}