feat: Only generate manifest if update is specified (#8)

* Only generate manifest if update is specified

BREAKING CHANGE: packwatch will now exit with a non-zero code if run
without an existing manifest file and without the --update-manifest
flag. Now run packwatch --update-manifest to generate a manifest for the
first time.

* Fail but still generate manifest

src/index.js

@@ -33,7 +33,9 @@ if (!existsSync(MANIFEST_FILENAME)) {
     console.log(
         `Package size (${currentStats.packageSize}) adopted as new limit.`,
     )
-    process.exit(0)
+    // If the update flag wasn't specified, exit with a non-zero code so we
+    // don't "accidentally" pass CI builds if the manifest didn't exist
+    process.exit(isUpdatingManifest ? 0 : 1)
 }
 
 const previousStats = getPreviousPackageStats()