Add TypeScript config for Astro website

Add tsconfig.json extending astro/tsconfigs/base and env.d.ts with
Astro client type references to resolve import.meta.env type errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/env.d.ts

@@ -0,0 +1 @@
+/// <reference types="astro/client" />

website/tsconfig.json

@@ -0,0 +1,3 @@
+{
+  "extends": "astro/tsconfigs/base"
+}