fix: Lower the supported NodeJS version from 18 or greater to only 18

Node 19 is [not supported by tree-sitter yet](tree-sitter/node-tree-sitter#127), and that is a dependency of ours. Until tree-sitter works on Node 19, we should not list Node 19 as supported.

Author: JakeChampion

package.json

@@ -2,7 +2,7 @@
   "name": "@fastly/js-compute",
   "version": "1.6.0",
   "engines": {
-    "node": "16 - 19",
+    "node": "16 - 18",
     "npm": "^8 || ^9"
   },
   "license": "Apache-2.0",