When the user configured `pageExtensions` with values beyond Vite's
defaults (e.g. `["platform.tsx", "tsx", "mdx"]`), extensionless imports
of files with those extensions failed to resolve and the build crashed
with `Custom deploy script failed: undefined (1)`.
Mirror Next.js / Turbopack `resolveExtensions` semantics: prepend the
configured `pageExtensions` (each prefixed with `.`) to Vite's default
extension list, preserving the user's priority so e.g. `.web.tsx`
resolves before `.tsx`.
Ported from Next.js: test/e2e/app-dir/resolve-extensions/
Refs: cloudflare/vinext#1502