Explicitly require a public repository for Stage 1

[mathiasbynens]

Stage 1 already had various entrance criteria (such as prose outlining the problem, the shape of the solution, examples of usage, the high-level API, etc.), which in practice are expected to be made accessible through a publicly available repository.

This patch explicitly makes the repository a requirement for Stage 1 advancement, resolving the following issues:

• Proposal repo should be a requirement #24
• Proposal repos: not required? #25
• update requirements for stage 0 and stage 1 on the agenda document agendas#722

[sffc]

Worth linking to https://github.com/tc39/template-for-proposals?

[ljharb]

Probably better to link to https://github.com/tc39/ecma262/blob/master/CONTRIBUTING.md#new-feature-proposals and then add the template link there if desired.

[ljharb]

I've added this to the agenda, so we can get consensus on it next week: tc39/agendas@205e270

[ljharb]

This received consensus today.

[DerekNonGeneric]

Just wanted to give you all a heads up that the process document doesn't seem to currently link to the proposal template repo.

Linking to the template repo in this document may help w/ the user journey of the website, which links to the process document.

Anecdotally, I thought I had heard of this repo, but was only able to locate it by searching this org's public repos for it. :)