Fix TS issue

Author: mnapoli

src/types/bref-layers.d.ts

@@ -0,0 +1,3 @@
+declare module '@bref.sh/layers' {
+    export function layerArn(region: string, phpVersion: string, platform: 'x86' | 'arm'): string;
+}

tsconfig.json

@@ -10,7 +10,8 @@
     // Useful for debugging (stack traces)
     "sourceMap": true,
     "resolveJsonModule": true,
-    "strictNullChecks": true
+    "strictNullChecks": true,
+    "types": ["node"]
   },
   "include": ["src"]
 }