.npmrc: engine-strict (#24217)

Enforce the existing `engine` directives requiring node at `16.x` and npm at `7.x`, failing with a clear error message if a mismatch exists:

```
npm ERR! code EBADENGINE
npm ERR! engine Unsupported engine
npm ERR! engine Not compatible with your version of node/npm: undefined
npm ERR! notsup Not compatible with your version of node/npm: undefined
npm ERR! notsup Required: {"node":"^16.10.0","npm":"^7.24.0"}
npm ERR! notsup Actual:   {"npm":"8.5.0","node":"v16.14.2"}

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/jesse/.npm/_logs/2023-08-08T15_09_40_758Z-debug-0.log
```

This change makes the version mismatch exceedingly and immediately clear instead of the developer having to infer from an obscure error much later on in the installation process.

webapp/.npmrc

@@ -1,2 +1,3 @@
 save-exact=true
 legacy-peer-deps=true
+engine-strict=true