feat(create-expo): support using github.com/ URLs without protocol

[EvanBacon]

Why

• make the github template option a bit more resilient.
• this PR also improves the error messages so you never see a "Only GitHub repositories are supported." when you provide a github URL.

Test Plan

• This works cead -t github.com/evanbacon/apr24-template --no-install
• This throws a reasonable error: cead -t github.com/evanbacon/test-missing-repo --no-install

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).