Document need for engines in package.json

[geoffrich]

As of 7 days ago, SvelteKit now uses Node 15+ syntax ||= at build time: https://github.com/sveltejs/kit/blob/616ec7acace562b4517e1dae6c2fab159f055b78/packages/kit/src/core/sync/write_types/index.js#L510

This caused issues when updating the demo app to the latest adapter in geoffrich/sveltekit-azure-swa-demo#8, since the build uses Node 14 by default:

 error during build:
SyntaxError: Unexpected token '||='
    at Loader.moduleStrategy (internal/modules/esm/translators.js:145:18)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! sveltekit-azure-swa-demo@0.0.1 build: `vite build`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the sveltekit-azure-swa-demo@0.0.1 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /github/home/.npm/_logs/2022-09-13T00_05_13_864Z-debug.log

I fixed this by specifying engines in the project's package.json:

	"engines": {
		"node": ">=16.7 <17.0"
	}

This should be documented in the adapter README. Before that, we should verify that this is the correct solution and there isn't a better workaround.

[geoffrich]

Thought: should this be fixed upstream in create-svelte? i.e. the default project template set the expected engines.

[geoffrich]

It's worth noting that the ||= syntax is no longer present in the build code, though that's not a guarantee that similar syntax won't be added in the future.

[geoffrich]

I'm closing this since it no longer appears to be necessary. I removed it from the demo app in #104 and it still deployed. Can re-open this if we need to address it.