add package.json engines and corresponding .nvmrc (#68)

I wish the nvmrc were not necessary, but it still is.
See nvm-sh/nvm#651

I think nvmrc is the most popular Node version manager, so it _probably_
makes sense to add the `.nvmrc`. If we want to skip this and go just
with the `package.json`, that's fine too.

Author: chadoh

.nvmrc

@@ -0,0 +1 @@
+17

package.json

@@ -30,5 +30,9 @@
     "eslint": "8.14.0",
     "eslint-config-next": "12.1.5",
     "typescript": "4.6.4"
+  },
+  "engines": {
+    "node": "17",
+    "npm": "8"
   }
 }