add .env.template for configuration and update token file path handling

.env.template

@@ -0,0 +1,7 @@
+CLIENT_ID=<discord application client id>
+CLIENT_SECRET=<discord application client OAuth secret>
+REDIRECT_URI=<your OAuth2 redirect URI, must match the one set in your Discord application>
+ESP_IP=<IP address of your ESP device>
+ESP_PORT=<port number your ESP device listens on>
+LISTEN_PORT=<port number your application listens on>
+TOKEN_FILE_PATH=<path to store OAuth2 tokens, relative to project root, or absolute>

src/tokenService.ts

@@ -1,5 +1,5 @@
 import { writeFileSync, existsSync, readFileSync } from 'fs';
-import { join } from 'path';
+import { isAbsolute, join } from 'path';
 
 export interface TokenData {
   accessToken: string;
@@ -8,7 +8,10 @@ export interface TokenData {
 }
 
 export class TokenService {
-  private TOKEN_FILE = join(__dirname, '../', '.discord-token.json');
+  private TOKEN_FILE =
+    process.env.TOKEN_FILE_PATH && isAbsolute(process.env.TOKEN_FILE_PATH)
+      ? process.env.TOKEN_FILE_PATH
+      : join(__dirname, '../', process.env.TOKEN_FILE_PATH ?? '.discord-token.json');
 
   public saveTokens(data: TokenData): void {
     try {