feat: add exist endpoint

[AmirSa12]

Not all packages on npmx have preview releases (i.e. not all packages use pkg.pr.new).
so, npmx can call this endpoint to check if a package exists on pkg.pr.new and render the button only when it does.

https://31157168.stackblitz-cr.pages.dev/api/repo/exists?owner=stackblitz-labs&repo=pkg.pr.new (true)
https://31157168.stackblitz-cr.pages.dev/api/repo/exists?owner=vitejs&repo=vite (false)

[continuous-releases-staging]

• example-1

• example-2

  npm i https://6cb0c4f9.stackblitz-cr.pages.dev/stackblitz-labs/pkg.pr.new/pkg-pr-new@550
  

  npm i https://6cb0c4f9.stackblitz-cr.pages.dev/stackblitz-labs/pkg.pr.new/playground-a@550
  

  npm i https://6cb0c4f9.stackblitz-cr.pages.dev/stackblitz-labs/pkg.pr.new/playground-b@550
  

commit: 0700e7d