feat: add type-checking for process variables

Author: lauslim12

examples/echo-bot-ts/environment.d.ts

@@ -0,0 +1,11 @@
+declare global {
+  namespace NodeJS {
+    interface ProcessEnv {
+      CHANNEL_ACCESS_TOKEN: string;
+      CHANNEL_SECRET: string;
+      PORT: string;
+    }
+  }
+}
+
+export {};