Update env usage for new conventions in Vite 4

There were a couple of improvements in the updated version of Vite
around decoupling build modes and NODE_ENV:

* vitejs/vite#10996
* vitejs/vite#11045

Accordingly, this change:

* Consistently uses import.meta.env.MODE to set build mode specific
  values vs relying on the (previous) behavior around how
  import.meta.env.PROD was set.
* Adds a development environment file so NODE_ENV is set to development
  for this builds.

.env.development

@@ -0,0 +1 @@
+NODE_ENV=development

src/lib/env.ts

@@ -1,10 +1,10 @@
 export const ENV = {
-  OS_RAISE_CONTENT_URL_PREFIX: import.meta.env.PROD ? 'https://k12.openstax.org/contents/raise' : 'http://localhost:8800/contents',
+  OS_RAISE_CONTENT_URL_PREFIX: import.meta.env.MODE === 'production' ? 'https://k12.openstax.org/contents/raise' : 'http://localhost:8800/contents',
   OS_RAISE_EVENTSAPI_URL_MAP: {},
   EVENT_FLUSH_PERIOD: 60000
 }
 
-if (import.meta.env.PROD) {
+if (import.meta.env.MODE === 'production') {
   ENV.OS_RAISE_EVENTSAPI_URL_MAP = {
     'raiselearning.org': 'https://events.raiselearning.org',
     'staging.raiselearning.org': 'https://events.staging.raiselearning.org'