feat: add Edge Functions workspace with tested email builders

Adds supabase/functions/ (deno.json, _shared/email.ts) with pure
buildBookingNotificationEmail/buildInquiryNotificationEmail functions
for Task 6's Edge Functions, tested via Deno's test runner. Excludes
supabase/functions/** from the root Vitest config, which otherwise
tried to resolve the jsr: import and failed.

supabase/functions/deno.json

@@ -0,0 +1,5 @@
+{
+  "tasks": {
+    "test": "deno test --allow-none"
+  }
+}