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>

tests/main-token-permissions-set.test.js

@@ -0,0 +1,7 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully sets permissions
+await test(() => {
+  process.env.INPUT_PERMISSION_ISSUES = `write`;
+  process.env.INPUT_PERMISSION_PULL_REQUESTS = `read`;
+});