feat: configure devJargons github app (#36)

This Pull requests a implements the DevJargons app within the
`src/lib/octokit/app` following the implementation done in #33, the
intention of this app is to have an bot representation that performs
some certain actions around PRs generated through the jargons.dev word
editor; this actions are example...

- Add appropriate labels to PRs generated through the Web UI word editor
available on jargons.dev.

This app is returned as an `octokit` instance that can be used to
performs actions as the github app on our project repo (because we have
given necessary permission).

### Screencast/Screenshot


![image](https://github.com/babblebey/jargons.dev/assets/25631971/d4c8a832-0fd7-473b-a36f-b13114053474)

Author: babblebey

.env.example

@@ -1,4 +1,8 @@
 # Create a GitHub App at https://github.com/settings/apps/new
+GITHUB_APP_ID=
+# IMPORTANT: private keys must be in PKCS#8 format, see https://github.com/gr2m/universal-github-app-jwt/#about-private-key-formats
+GITHUB_APP_PRIVATE_KEY="-----BEGIN PRIVATE KEY-----\nMII...und==\n-----END PRIVATE KEY-----\n"
+
 GITHUB_OAUTH_APP_CLIENT_ID=""
 GITHUB_OAUTH_APP_CLIENT_SECRET=""
 

src/lib/octokit/app.js

@@ -1,7 +1,18 @@
-import { Octokit } from "octokit";
+import { Octokit, App } from "octokit";
 import { createOAuthAppAuth } from "@octokit/auth-oauth-app";
 import { oauthAuthorizationUrl } from "@octokit/oauth-authorization-url";
 
+const app = new App({
+  appId: import.meta.env.GITHUB_APP_ID,
+  privateKey: import.meta.env.GITHUB_APP_PRIVATE_KEY,
+});
+const { data: { id } } = await app.octokit.request(`GET /repos/${import.meta.env.PROJECT_REPO}/installation`);
+
+/**
+ * DevJargons Helper App's Octokit instance
+ */
+const devJargonsOctokit = await app.getInstallationOctokit(id);
+
 /**
  * OAuth App's Octokit instance
  */
@@ -62,6 +73,7 @@ function getUserOctokit({ token, ...options }) {
 
 export default { 
   octokit,
+  devJargonsOctokit,
   oauth: {
     getWebFlowAuthorizationUrl, 
     exchangeWebFlowCode,