feat(react-email): Build preview app and run it built instead of as dev (#1537)

Author: gabrielmfern

.changeset/five-humans-dream.md

@@ -0,0 +1,5 @@
+---
+"react-email": major
+---
+
+Use a built version of preview app when running `email dev`

benchmarks/preview-server/bench-results-30-iterations.json

@@ -0,0 +1 @@
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