`@remotion/vercel`: Convert sandbox scripts to real TypeScript files

Sandbox scripts (render-video, render-still, ensure-browser) are now real
TypeScript files under src/scripts/ that are type-checked and linted.
bundle.ts reads them at build time and generates string-exporting modules
in src/internals/. Dynamic values (codec, outputFile, imageFormat) are
passed via JSON config arg instead of being baked into template strings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>